Hi Reggie - DD1 (now 10 months) also had reflux when she was a newborn, which was treated with Gaviscon, and she was also on Apatmil Comfort for colic. I can't say that either ever made her projectile vomit, but every baby's different, so maybe the Gaviscon isn't agreeing with your son anymore. Are you giving the Gaviscon with every feed? If i remember correctly, I think you're only supposed to give it 5 times in a 24hr period, but I'd double check that.
Is he projectile vomiting after every feed, or managing to keep anything down? When you say he's slept through do you mean he's drowsier than normal, or still his usual level of alert? Is his temperature normal, and is he doing plenty of wet nappies?
Sorry for all the questions, this is just a mental checklist I do myself when deciding on what level of worried I should be!
If he's otherwise his normal self, i would hold off going anywhere tonight, keep an eye on him over the weekend, and go to the doctors on Monday to see what alternatives there are. If you're worried about him getting dehydrated, you could try giving him a wee bit of cooled boiled water and see how he gets on, or just cut out the gaviscon altogether for a day and see how he gets on.
Aptamil also do a reflux milk, have you given that a go?