Hey M.. Our daughter is 14 months and also mildly hypotonic -- doesn't walk yet, does cruise furniture, has a hard time with sitting up etc. Our pediatrician suggested a pediatric neurologist and we went to see her, and basically she said that it wasn't severe enough to worry about ; that kids grow out of it and that if we didn't live in such a high-anxiety parenting zone we probably wouldn't even be OFFERED physical therapy, much less feel that we need to take advantage of it. Her advice was pretty much to just put stuff out of the way so the baby has to reach for it, to try and work with her to pull herself up, etc. -- but otherwise to wait it out.
That said, obviously if your son is severely hypotonic you should take action. And you might want to do the PT just to have someone else work with your son for a bit, so you can see what kinds of exercises to do at home with him. But if he's cruising and standing, I'd say you might want to relax a bit and accept that he's a bit of a late bloomer, motor-skills-wise.
By the way, I'm happy to send you a copy of the PT exercises we got from our pediatrician -- just send me a private message with your address.