Occupational therapy. Normal toddlers can be gaggy with certain foods. You will know if it is time for professional intervention if they begin having a feeding aversion and avoiding certain textures. When feeding time becomes a chore and you find yourself unable to cope or your child is not gaining weight it is time for help. Your pediatrician is the first place to start for advice and growth parameter evaluation.
Soft, mushy foods. In general soft, mushy foods can help minimize the effects of a hypersensitive gag reflex, however everyone is different. It will take some (messy) trial and error to find out exactly what he can and can't eat. A good pediatric speech therapist is a great resource, you can find one affiliated with your nearest children's hospital.