Your best bet is to keep a close eye on his nursing habits. If he loses interest in nursing or starts spending less time at each nursing session, that may be a cue that he's filling up on solids and is too full to nurse. If you pump, you can also make sure your supply isn't decreasing that way. It's a hard balance and requires some guesswork...I suppose that if he's ravenous by nursing time or is getting hungry early, you can probably increase his solids. Good luck!
