Yeah just go with the flow hun. Dont let it stress you out coz it will just stress him out. The best thing u could do is not to make a huge deal out of it. Its probably just a phase and hopefully it will pass.

Mayb try letting him sit on it, but dont do it up (watching him ofcouse) or let him play with it during the day (the booster seat).
If he still isnt warming to it, move on, and let him sit somewhere else. Kids are funny little buggers lol