Encouraging eating of solids - Growth hormone defiency

I have a 20 month old son who is being treated for growth hormone defiency. Before this diagnosis I had tried to get him to eat solids without much success. Instead he only wanted to nurse. His doctor thought it was because he full on breastmilk that he had no room for solids but even when I left for 12 hours a day he didn't want to eat anything. He was just not hungry.
Since he started treatment his appetite has increased but that only means he wants to nurse more. I don't want to suddenly make him wean but I would like to encourage him to eat more solid food. He does eat some solids but not that much and not enough for him to get enough nutrition without breastmilk.
My feeling is that his body never learned to feel a normal amount of hunger and so he missed the window where babies normally develop a taste for eating solids on a regular basis. I have not been able to find anything online and his doctors don't seem to know.
How do I get him interest in eating more solid food without a huge battle?