I don't think I'd replace the clip or anything like that... it's just time to teach him when it's okay to move the clip and when it's not... start encouraging him to do up the chest clip (and even do the buckle himself, with your help, if he can) and, once you tighten the seat, pull the chest clip up where it should be all by himself... and then praise him for being "such a big boy" and "now you're all ready for a safe ride"... then, when you have parked the car and are going to get him out anyway, say... "now you can get out of your carseat. Go ahead and undo your chest clip and I'll come help you"... so he can push it down, unbuckle it, and even start working on unbuckling by himself (unless you loosen first, then he can try after you loosen him)
Give him a sense of control and really praise him for being safe in the car... he'll catch on quickly.
When Ruthie moved to the Nauti, all I had to do was turn in my seat (she was behind the passenger seat), push the lever and she would lean forward to loosen it, then she'd undo the chest clip and I'd unbuckle it till she learned to do it. I could even buckle it for her that way (turned and using one hand) until she figured out buckling) and then pull the strap to tighten her.
When she was 3 and (although it was a different seat) was doing stuff like that, I didn't allow her to have freedom in getting in/out on her own at all... I made her let me LIFT her in, LIFT her out, I didn't loosen the straps till it was time to rebuckle her... UNLESS she'd been good the whole trip. If she hadn't, then she didn't get to do it herself, because she wasn't being safe by herself.
Also, drawing on her hands (with washable marker) worked really well... she was all about caterpillers and butterflies and bugs... her favorite was for me to do a flower on one hand, a butterfly on the other, and she'd talk to her butterfly and take it flying to the flower to eat.
I'd also think about why he did it... was it a long trip?
One to a place he was really excited about?
I'd offer little helpful reminders in those instances (along with things to keep his mind busy), like... I know you can't wait... don't worry, we're almost there