This is a problem we faced for a time with my now three year old; not exactly the same (we had a problem with him getting out of the harness/unbuckling it rather than loosening it), but works out to the same thing. By two and a half or so he was capable of both unbuckling himself and buckling himself. (This is with a new-when-he-was-1 Evenflo carseat that's quite difficult to unbuckle - he's just that determined.)
We dealt with it in three ways. First, we explained in detail why he had a carseat and why it was important for it to be buckled and reasonably tight. I at one point showed him a toy bus with a 'Little People' figure in it, and caused it to have a crash (ejecting the figure). We didn't do this in such a way that it scared him particularly, that wasn't the intent; it was intended to convey the information.
Second, we have gone to a few auto shows with him (since he was 1 1/2), and one of the things that we found is he likes sitting in the front seat of the cars and pretending to drive. We took this and ran with it. Occasionally, when we have some extra time, we let him sit in our car in the front (usually driver's) seat, and pretend to drive. One of the things he does - and he does this on his own, but if he didn't we would've pushed it - is buckle himself in before he starts pretending to drive. It's not perfect - he can't really reach the steering wheel buckled, so what it ends up being is he buckles himself in, then he gets out - but he really likes buckling things.
We also let him buckle himself into the carseat. At 2 - 2.5 he would buckle 'top' (meaning the little plastic bit that goes at the rib cage); he's only allowed to do this if he actually does a good job (ie, buckles himself in regularly and doesn't unbuckle). We initially prompted him to do it (by asking if he wanted to), now we mostly don't but just verify. Now, at a bit over 3, we allow him to do all of the buckling - again, as long as it's done right - and verify (including tightness) afterwards. This gets some 'buy-in' to the process from him, and is a big 'treat' to be given the responsibility.
Third, when he did unbuckle himself, we immediately (as safety allowed) pulled over to the side of the road or exited the highway, re-buckled him, and reminded him of the safety issue. Only once did we ever have an issue where it needed more than one stop; that time we just sat on the side of the road for several minutes until he stopped unbuckling. We made it clear it was annoying to have to stop, but didn't yell at him or punish him - that didn't seem like it would be effective.
Ultimately we see this as a part of the whole seatbelt experience that he'll continue to have as a teenager, and hope that treating it this way helps him to make good decisions later in life in the same manner.