He'd get much better exercise in a swimming pool. I would start exploring your options to get access to a pool. Do you have a Y where you live? They offer scholarships and financial aid. They would also have a hot tub. His physical therapist may have other ideas where he might regularly access a swimming pool as well.
Is the PT not setting him up on a home exercise program? That's very odd. He's certainly capable of following directions and doing home exercise. It is very rare for someone to go to PT three times a week. For similar issues we were going once every three weeks and working a daily home exercise plan.
In terms of building a home exercise room, one thing I'd keep in mind with hypotonia it isn't like he needs expensive equipment to get a good work out. You could probably get everything you need for him to be exercising at home for less than $25 and certainly less than $50. Again the PT could help with suggestions including therabands (they would probably give those to you for free), inexpensive hand weights - you could even substitute soup cans, a therapy ball - sold at places like Target for $20, using free stuff like the wall and stairs.
It is really more about motivation and knowing the appropriate exercises than about having equipment. If you do decide to get something like a treadmill, check craig's list and freecycle and the ads in the newspaper. Most people who buy expensive exercise equipment don't end up using it, so you should be able to get some cheap. I would urge extreme caution though in purchasing equipment. Using weight machines with hypotonia offers a lot of possibility of getting hurt. That really needs to be planned very carefully. He could get much further with one or two pounds weights and resistance bands.
