Ditto bpatters. If he doesn't particularly like the food, it won't take him long to figure out that if he doesn't chew it, you will take the syringe out and not give him any more.
In my limited experience, their mouths will hold about 1/3 of a 1 cc syringe's worth of food at one time. Go deep near his molars and depress the plunger VERY SLOWLY. Don't try to give more than about 1/4 to 1/3 of the syringe at one time. Wiggle the syringe if needed a bit to get him to chew. Then, once he's chewed and swallowed, give him another 1/4 to 1/3.
Don't remove the syringe until it needs to be refilled.
If a guinea pig is chewing, he is swallowing. (If you watch a cavy drink, they actually chew their water.)
Be SURE you are using a 1cc syringe with the tip cut off and the rough end smoothed!
These instructions will NOT work with any syringe larger than that.
If he aspirates, he will cough violently. Of course you don't want to do that, but generally if a guinea pig aspirates there's no question as to what has happened.