Non solid suggestions for 3 year old non verbal autistic boy

My son was diagnosed at 1 1/2 and has been getting speech with feeding but I feel like after a year of therapy he still hasn't improved with solid feedings he gags when I try to give him anything  solid ...he only eats yougurt , a thick Spanish oatmeal milk that is mainly what he drinks through out the day  and the Gerber baby food banana. Can some one tell me how they get there little one or loved one to eat solids if they are going through this ...much love hope all is well thank you for reading