My now 16 month old did the same as your son but after she self-weaned while we were on vacation at 14 months, then all of the sudden, she is a cow drinking machine! At one point, I was really worried about her intake of "healthy" stuff but I just kept putting a little cup of milk there with her water or juice at least once a day.
Also, I still take stage 3 foods and add some milk and she will eat them or I mash them up some more and tell her "you have to chew chew chew". She started eating pasta one day because I was having a plate of pasta when she was eating - she reached for a piece and from then on added pasta to her likes! Same thing with pizza - we were out with a friend whose kids were eating it and poof - now she loves pizza.
You are being a great mom with the breast feeding and being concerned - just hang in there... he is still getting a lot of nutrients from your milk and will pick up the rest of the stuff in time. My husband and I used to joke that our daughter would need to take a blender to college with her because she still wouldn't be eating solids....
Good luck and follow some of the advice from other moms.
blessings,
stacy