N.P.
whenever you are ready to feed him his jar food, put him in the highchair and put some cherrios or kix cereal or other easily grabbed finger foods on his tray. Goldfish are good too. Canned veggies and fruit are good. Only put about 4 pieces on there. Let him experiment with them as you fix his jar food. If he gags, don't react to it. Only react if he has a positive thing done like putting some in his mouth or grabbing it. You can even put crackers on his tray or pieces of bread. Most of it will not make it to his mouth, or he may put it in his mouth then spit it out. All this is normal and ok. It's part of the process.
It may seem like it will take forever, but seriously it happens so quick if you just make it part of the routine.