Some babies do get the hang of bfing efficiently more quickly than others. If he is bfing well and gaining weight, etc, I wouldn't worry about the length of his bfs, more with the number that he's having. Especially if you feel your supply is bountiful. It's normal for the rush of oxytocin to make both mum and bub feel sleepy (I lost count of the number of times I slept blissfully in the armchair when DD was tiny!).

Have you tried bfing him without the shields now you've been doing it for a while? Some mums with flat nipples find the shield helps to draw it out and once this has been done and baby is a bit bigger he can attach more easily.