
You may have over exercised him, till mature [this varies by breed but at least 12 months old]walks should be limited to 5 mins per month of age, so in your case 15 mins away from home and 15 mins back, any other exercise should then be limited to the garden, not repetitive like ball fetching more than a couple of times, obedience training will tire his brain without putting stress on his joints.
Don't be pressured into neutering him as the sex hormones influence bone growth amongst other things not related to reproduction.
Hopefully he has just twisted something and with rest and no outdoor walks it settles down.