We took Boots to the vet and it seems like he has something called bicipital tendinopathy.
Sometimes dogs develop a foreleg lameness without any witnessed injury. Initially, you may note only stiffness or limping when rising or after heavy play. Over time, the limping becomes consistent. One of the possible reasons for these subtle changes progressing to a lameness is tendon damage, which can range from uncomfortable to painful.
We just need to rest him and give him some anti inflammatory drugs.