Are you giving him finger foods? Just let him explore and don't hover too much. He'll get it, but you have to give him the chance to do it. Give him big chunky things (chip-sized food). He can't actually swallow those whole and will have to at least bite off bits, which will eventually lead to chewing. If he has good use of his pincer grip, then small food like peas or cubes of carrot or easy to mush food like mashed potato also are great. Honestly, my daughter doesn't always chew her food. We gave her raisins the other day and then next morning about 40 whole raisins came out in her nappy! But there's a difference between not chewing and gagging/vomiting and you might just need to give him time to figure out how to safely chew and swallow his food. The best thing is lots of finger foods (not chunky or mashed food on a spoon as it doesn't let him have as much control over chewing and swallowing) and just giving it time. It's an in-borne instinct in babies to be able to eat safely. He may have an especially strong gag reflex, but that's usually gone in most babies by about 7 months or so. I think just be patient and let him take the lead on what he needs.