You mentioned he might have parasites. Did the vet give you a de-wormer? Usually one or two doses of Drontal does the trick, and it's an inexpensive drug.
Can you leave dry food out for him 24/7, and supplement with canned food once a day? If he does have parasites, he will be very hungry, and possibly anemic.
Have you tried getting him some catnip toys? Cats seem to love those. Also, you can try to engage him with a wand toy and a laser toy.
I would advise you keep him indoors. My guess is that once he's neutered, he will very happily settle indoors, because the instinct to roam and mate will leave. It may take a few weeks after the neuter for his hormones to settle down. Another option would be to take him out on a lead and harness--this is what I do with my one cat who loves to go outside. It takes several trips outdoors for a cat to get used to being on a lead, and they don't usually walk beside you like a dog--but my cat loves going out on a lead, and yours might, too. I can tell you from experience that it's much less stressful caring for an indoor cat than an outdoor cat, because you worry a lot less. Since this is a stray and not a feral, he can most likely adjust to life indoors with walks outside on a lead. I can give you some advice on that when he's healed up from the neuter.
Can you leave dry food out for him 24/7, and supplement with canned food once a day? If he does have parasites, he will be very hungry, and possibly anemic.
Have you tried getting him some catnip toys? Cats seem to love those. Also, you can try to engage him with a wand toy and a laser toy.
I would advise you keep him indoors. My guess is that once he's neutered, he will very happily settle indoors, because the instinct to roam and mate will leave. It may take a few weeks after the neuter for his hormones to settle down. Another option would be to take him out on a lead and harness--this is what I do with my one cat who loves to go outside. It takes several trips outdoors for a cat to get used to being on a lead, and they don't usually walk beside you like a dog--but my cat loves going out on a lead, and yours might, too. I can tell you from experience that it's much less stressful caring for an indoor cat than an outdoor cat, because you worry a lot less. Since this is a stray and not a feral, he can most likely adjust to life indoors with walks outside on a lead. I can give you some advice on that when he's healed up from the neuter.