If you've already sent him a message, wait until you hear from him.
If he didn't get it, he would have told you by now unless he died, has amnesia or was committed to a mental institution.
Some people either just don't cash checks right away or maybe he put it somewhere and forgot it.
Just consider it spent and deduct it from your spendable balance.

If you sent him a message via ballreviews, check your listings and see if he read it, If not,  you can always send him a letter to the address to which you sent the original check telling him to cash it; so you can better balance you check book. If he has read it, there's nothing left to do.