Seeing as you only have one solid meal after he wakes up from his nap, what about offering a snack on wake up from the afternoon nap and then offering dinner a little later, say 5.30pm. If you could do this without impacting on BT it might be worth a try. If you think this won't work for your LO, perhaps offer a snack before bathtime and milk and Bt routine. What do you think?
Having said all of that, LOs do go through phases of needing more milk, has his solids intake reduced recently, eg from teething pain? When my DD is teething, she tends even now to eat less in the way of solid food and take a bit more from her milk. When she was under one I went with it and added more formula - if she took it I let her you know.