Pain2 electric boogaloo

For starters, while Max was originally deformed in multiple ways, I absolutely think that the Entity exaggerated his features to appear more monstrous, probably for her own amusement, and for the terror of the survivors (maybe also to shame or humiliate him for some reason). Also maybe inbreeding. His parents were deep town farmers in the early 1900s.

Eyes: Severe cataracts and Aniridia (In Aniridia, the iris of the eye may be completely or partially missing, so you only have a pupil. This, combined with severe cataracts, could lead to an eye that appears entirely, or almost entirely white). Alternatively, it could be an extreme case of Corneal Leukoma, where a spot(s) appears in the eye, partially or completely covering the iris and pupil in a porcelain looking whiteness.

Another possibility could be extreme sunsetting eyes caused by Hydrocephalus (fluid in the brain), which could also lead to the conical shape of his head (more later).

Hair: Naturally thick, coarse hair, mixed with conditions such as Hypothyroidism or certain vitamin deficiencies, could cause the thick, sorta horse-like hair that grows down the hillbilly's neck and upper back. We can also see by the farm that the area he lives in seems very dry, which can worsen texture.

Body shape: Most definitely severe Congenital Kyphoscoliosis, a type of scoliosis where the spine bends both sideways and forward, leading to a possible hunchback and thrust torso look. Could also be combined with Pectus Carinatum (pigeon chest).

His ribs expand outward, which would most likely just be caused by a rib flare, and malnutrition can make it look much worse.

For the growth on his back, it could be a lot of things: a benign tumor, a cyst, a lipoma (fatty benign tumor), an abscess, a massive lymphoma (cancer of the lymph nodes which can cause tumors all over), a soft tissue sarcoma (fleshy or fatty tumors) , lymphedema (an obstruction of the lymph nodes causing swelling of lymph fluid), a huge keloid, and more- largely depends on the texture of the growth, with a soft, squishy texture being most of the above benign growth, and a hard texture being most of the above malignant growths.

And like I mentioned early, the conical shape of his head could be a somewhat mild form of Hydrocephalus, or something else like; Deformational Plagiocephaly, where a newborn spends too much time with it's head resting in a certain position and the skull changes shape, Carpenter syndrome (malformations of the skull bones), or Craniosynostosis, where the skull bones fuse too early in development, forcing the rest of the skull to continue growing a different way. Of course there are some others as well.

Aside from the above, the shape of his body is fairly normal.

Body/face abnormalities: These on the other hand, account for basically every strange things about Max's body.

We can very clearly see that the skin on Max's head and upper body tends to stretch and connect to other sections of his body. This could be an extreme, extreme case of congenital skin bridging, where sections of skin, most commonly on the neck area, connect and bridge to other sections of the body, making it look like someone punched out a hole of flesh. Yum.

The areas of smaller growths and bumps could also have a lot of different causes. Neurofibromatosis type 1, for instance, can cause uncontrolled tissue growth along the nerves, leading to what can look like extreme skin tags. Tuberous Sclerosis does something similar, but acts as more of a combination of NF type 1 and type 2, possibly causing these non-cancerous growths among all vital organs, including the skin (I find this one more likely based on the way these growths look on Max). Leprosy would make a lot of sense, and account for some of the deformations we see as well- so would Hidradenitis Suppurativa (Acne Inversa) which causes large, generally pus-filled lumps, mostly along areas with lots of skin folds (ie, groin, armpits, stomach, under breasts, etc), and can cause large, weeping sores/open wounds which can easily get infected.

The general texture of Max's skin looks pretty leathery (whether or not it actually feels leathery), as well as hyperpigmented. The former could be caused by Scleroderma (the hardening and thickening of skin caused by autoimmune disease), Diabetes, Ichthyosis (group of skin disorders acting similarly to scleroderma), Pseudoxanthoma Elasticum (progressive mineralization and fragmentation of connective tissue and elastic fibers, causing a leathery look), Lichen simplex (patches of dry, itchy, leathery skin), and more. However, a *combination* of the hyperpigmentation and leather-ness could be caused by things like severe sun exposure and chronic sunburns (he is a farm boy, but he was also locked up in a basement for like 20 years), Cutaneous Amyloidosis (Amyloid deposits in the skin cause it to become thick, hyperpigmented, and leathery), or maybe Acanthosis Nigricans (often from insulin resistance, causes thick, velvety, sometimes leathery skin).

We can clearly see areas of Max's skin that have split open, causing deep wounds that he tries stitching back together with wood staples or something, which makes me think his skin might be brittle, another things that can be caused by the above skin disorders. He could've also just slashed himself open with his chainsaw or a sharp corner.

As for the hard plugs on one of his arms, these are most likely to be caused by Nevus Comedonicus, closely grouped papules that resemble giant Comedones- or blackheads. However, in this disorder, the comedones can actually get this massive in real life, and occasionally look worse- such as being striated, gaping, or covering an entire limb.