It's milk that's caesin-based (usually!) so harder to digest.

However, do make sure you check it out as there have been moves to put leptin (the "I'm full!" hormone) into baby milk so baby doesn't feel hungry. That's just dangerous for so many reasons. While I don't particularly like that DS is finding things hard to digest (but do like I get more than one hour's sleep at night) at least I'm not stuffing up his hormones already!