It's unlikely that they'll drop beyond about this age - but, I guess, still technically possible if his inguinal canal were to close later than normal.
Retained testicles carry with them two risks - one is a high chance of turning cancerous, the other is the possibility that they might wrap around internal organs. The former is a relatively high risk, but it's very unlikely that it would happen at a young age (it's more of a consideration for those who're wishing to keep the dog intact), and the latter is perhaps something more likely to afflict a young dog, but still overall a low risk in the near future. In short, I don't think there's any pressing need to rush ahead to have him neutered immediately. Waiting a couple of months, just in case, seems quite reasonable to me.
