Isn't it interesting that Stan never boasts about the things he actually did? He lies about how great he is with women or how he tried to save the pig in an epic battle against the pterodactyl (the first time). But when he actually fought the flying beast, or protected the kids from an army of zombies or managed to escape from the special agents while floating around with zero gravity he never mentioned it again. Save to say, unless the kids told Ford about it, he doesn't know what Stan is capable of. Of course he trusts his brother after Weirdmageddon and goes on adventures with him but I believe Ford still sees himself as the protector and fighter between them two. After all, he has a lot of experience and kept his body at peak health while Stan loves his Ice cream dinner and watching TV.
Imagine Ford trying to protect Stan from a mystical sea creature that attacks the boat but Stan just jumps in and obliterates the thing within a second, because why wouldn't he? It threatened his brother! Maybe not as efficient and clean as Ford would have done it but nevertheless. And the only thing he does afterwards is complain about his back, telling Ford he should clean up the mess, and goes back to whatever he was doing before. Leaving Ford on the deck, who tries very hard not to short circuit.