Hi there,
I'm having similar problems with Kyla and solids - although she is ok with drinking her milk.
Kyla is not at all interested in solids - the pead told me that basically she will eat when she is ready to and just to keep offering it to her. She is also involved in a prem study and they suggested stipping her down and letting her play with her food.
I do understand how frustrating it can be and worrying.
As the others have said - as long as he is putting on weight and not loosing, try not to worry too much.
You may also get some advice on Austprem - from people with older bubs who have had similar problems