The trick with broiler mixes or broilers, straight, is that you have to monitor their food intake. Unlike other breeds, it's a good idea to give them food part of the time and not the rest of the time - the rest of the time they should have exercise and be able to peck at stuff to keep them busy. Bits of shiny chain hung from their cages if they're caged, things to entertain them if they're in a run, or preferably grass range when they're old enough.
And if you want them to be pets, definitely don't use broiler grower.
They're not really designed to live past the few months it takes to slaughter them. Even if mixed, that comes through into the mix until you breed it out. So you have to take special measures and know that you might still lose them.
I raise the turkey equivalent, Broad Breasted Whites - definitely not meant to live past a few months. But stupidly I've become attached to them (grin) so I have five pets. They're the same - grow fast, their systems run out soon, they have leg issues, etc. So mine are totally free range with grains and crumbles available and lots of exercise.
As for aspirin, it's a blood thinner. So it's not what you want to use long term or he will just wear out. Instead, let's see if there's another way to keep him fit and moving about.
What does he have available to eat, where is he kept (range, coop, run, etc), and what is his general life like?