My boy was also pretty slow to fully recovery. I saw a lot of steady progress in the first few weeks, and then I felt like he kind of stalled - not getting worse, but didn't seem to be getting better. In time he did, though (it was just slow). It was a good 6 months before he seemed to be 100% recovered.
Definitely follow up with your vet if you are concerned, but hopefully he's doing ok. My mom's boy also had the same surgery and she felt like he wasn't progressing as fast as he should have been, but everything was fine in his case as well.
(Just to clarify - my boy had two problems at the same time - luxating patella and ruptured ligament plus he'd already developed arthritis, so his surgery was a little different and might have had a longer over all recovery than a typical ACL repair. I don't think my mom's boy took a full 6 months to recover, but I do know that she went through exactly the same thing you are - not seeing progress and worried that the "repair" had torn.)