The ACL is not a "joint" - it's a ligament. While there is no harm - and indeed a lot of good - in giving joint supplements, it will not fix or even help the ACL.
ACL's will not fix themselves, and yes the surgery can be expensive and a proper recovery requires weeks of VERY limited movement (crate rest with only brief respites for SLOW controlled walks for WEEKS).
Strongly suggest that you find an orthopedic surgeon who has successfully performed this surgery. A veterinary college is a good source to find one in your area. I would not just leave it though. Have a good surgeon look at it and give you his best opinion as to if he suspects ACL or not.
And as heartless as it sounds, I don't recommend pain meds either. Pain is the body's way of telling itself "don't DO that!" and dogs are fantastic at reading their bodies. If you give him pain meds he might continue to do the things he's not supposed to. I typically don't medicate my dogs for pain unless I've already taken steps to fix whatever the problem was that caused the pain (ie broken bone is now splinted).
I know lots of dogs that have undergone ACL tears - it's really quite common especially amongst canine athletes (agility in particular). Have had one of my dogs done in fact (successfully). Most that I know of are successful - the KEY is the restricted mobility during the recovery period - DON'T LET THE DOG RUN, JUMP OR CLIMB! Short, slow walks just a few times per day and CRATE REST the rest of time. I would let mine out of his crate to join me on the floor for a cuddle and ear rubs, but he was on a lead so he could not go further than two feet.
Good luck.