Not sure tbh but you could always experiment. At this age Imogen was at nursery full time so it was hard to know what size portions she was eating, but she was having a boob in a morning, 3 bottles of about 4-5oz ebm, an after work and a bedtime boob too - so a lot more than Noah's having. She was also having a morning snack - usually toast which, in the beginning at least, wasn't so much eaten as returned to dough! - then a lunch which would be a pureed version of whatever the bigger kids were having, and dinner in a similar vein.
This doesn't mean either way is right or wrong, cos as you know there are no rules on this. I would have thought he maybe needs a bit more calcium because he's still growing - bone development and all that, but if he has a lot of yoghurt and cheese, these count as milk too.
You could try experimenting with different portion sizes and timing etc to see if that encourages him to have more milk, but it could be that he's ready to leave the booby?
Sorry that's in no way helpful, but possibly a little reassuring?
xx Em