I agree that it would not hurt to double check with him, on the off chance he hasn't kept track of the fact that this check is outstanding.
It's good he gave you a head's up and post-dated the check. One of our wedding gift checks bounced, and the bank we were with at the time charged us
a bounced check fee because of it, even though no check we had written had bounced as a result of this.
I never did address this fact with the givers, because, what could I possibly say that would not embarrass them? So, we just wrote it off and ate the fee. In a way, though, they did me a favor, because it was the final straw that made me leave that bank. That bank was ahead of the curve as far as charging a fee for every little thing; this was mid-nineties, and although they mostly all do that to some degree now, such was not the case 20 years ago. I certainly wasn't too happy about the whole mess at the time, though. (Then the same thing happened with a birthday gift check to my son, but this time it was from my sister, so I was able to tell her about it easily. she gave me cash, and we had a good laugh. Plus, we had switched from that terrible bank by then, so no additional fees were incurred as a result.)