If the strap is uncomfortable, there are cushioned strap covers that can make it less "owie". But if it's just that he's not happy with being buckled in (as many 2 year olds are not)try instituting a reward system for leaving the straps as they are supposed to be.
For a two year old something as simple as a single M & M (there are health-food store equivalents) will be a sufficient reward for compliance. Even a two year old can understand: That's not safe. If you stay safe by leaving the strap alone then you get this! yay!(I"m not a big candy proponent, but this is a major safety issue. The expectation and habit developed will have a longer lasting impact than simply saying "no." Another option is having a visual checklist posted next to him (you can use velcro) and every time he obeys you (with one reminder) you can check it off. Tell him if he gets three checks then he gets....whatever you have decided ahead of time. There are many different reward options you can give him for compliance: 15 minutes of video at home later (with you), reading a special story together, giving him a special (healthy but yummy) treat.
Another strategy is to have a special toy that he can only hold when he's in his car seat (which makes it more attractive). Explain it to him, buckle him in and then hand him the toy. We call this "first, then..." strategy. FIRST you get buckled in and THEN you get the toy. If he reaches for the strap, gently take the toy back. Calmly remind him, As soon as you settled down you get to hold your toy. First you get buckled up and then you get the toy. If you don't buckle up, you don't get the toy. Usually kids will go right for it. If not, be calm (I know, I'm sure you're in a hurry, but it won't take long for him to catch on I promise) and wait him out holding the toy until he settles down. Praise him a lot for making good choices.