I agree with artyfarty you need to put him down before he gets over tired. So as soon as he starts yawning, rubbing eyes, whinging etc. At his age he will prob spend 2 hours awake but after that, need a nap.
Put him down in his moses basket, give him a dummy, put his mobile on then go out the room...If he cries go back in, talk softly to him, rub his tummy etc, he should calm down...