Of course Mike wouldn't be able to go near Will after volume 1 because even the touch from Mike's hand could snap him out of the mindflayer's possession.
He tried to run to him here. He was the first to get to him but he still was the furthest away when he sat next to him. You can see his hand going towards Will's arm, but he immediately brings it away. It's as if he wanted to put his hand there but something stopped him.
He was so worried for Will. He even chose to stay with him while the other characters were doing other things related to the plan. He seemed like he was about to go over to him, or to say something. He couldn't though, for some reason. All he could do was stare as Will was laying there on the couch.
He saw Will left behind. He was being attentive to him like always. He was caring about him and once again worried for him. He turned around as if he would stay with him, but didn't. He reached his hand out to touch his arm, but it didn't actually reach him.
He never even touched him at all in volume 2, which is strange, especially for Mike who'd always show some sort of affection towards Will in that way, especially when worried. He kept trying to, but never actually did.
Like in season 2, Will is literally in a Vecna trance and somehow Mike's hand on his shoulder alone can break him out of it.
Even Mike's words can get him out of the mindflayer's possession. All he has to do is say something heartfelt to Will and the mindflayer no longer has the control.
The second Mike leaves Will's side and stops caring about him is when the mindflayer gets to him. If Mike talked to Will or came close enough to him or even showed any signs of having love for him, the mindflayer would have no power over Will.
Mike had attempted multiple times to be close to Will and take care of him after volume 1, but each time, he failed. The mindflayer wouldn't let him get that close, because it would ruin Vecna's plan. (This also makes sense with what happens in The First Shadow, where Henry couldn't touch Patty at all because it would take away the control from the mindflayer).
Potentially though, Mike actually did have moments with Will, but they were erased from his mind immediately after so that the mindflayer can stay in control (if you want to know more about my theory on that). If Will figures this out, he could break the possession.
Another thing to add is that it was mainly after November 6th that Mike couldn't touch him anymore. On November 5th, he tried a little harder, with "come back to us Will" which I always found weird how that line was delivered, as if he was being stopped from doing more. It felt almost like it was his inner monologue talking.
After November 6th, it seemed like he kind of stopped trying. He had no power to be able to go near Will anymore. This goes along with playgate where Vecna is controlling everyone like puppets.
I think Will got possessed after finding his powers in episode 4. I think he was kinda possessed for like basically the whole season actually, but after gaining powers, the mindflayer took full control over him. Vecna then started erasing his memories, and having Will spy for him without a choice. Now he's controlling everyone and making sure that Will doesn't get the love he wants from other people so that the possession never breaks.