I would use an Ivermectin, like Equimax, before I used the Quest or Quest Plus. Normally I'd say have a fecal egg count done before deworming, but since he's from a rescue, he's likely loaded and you need to reduce the load slowly. Quest is a VERY efficient dewormer and will kill off a big load all at once which can cause an impaction colic. If you do Ivermectin then it's less efficient but will get rid of a lot but less likely to cause a problem. When I rescued horses (back before the fecal egg count) the rule of thumb was 1/2 dose of Ivermectin if the horse wasn't emaciated. So if the horse weighed 1000 lbs, give him a 500 lb dose of dewormer. Then in a week, repeat, and 2 weeks after that a full dose and 1 month after that a full dose of Quest, just be sure you have a fairly accurate weight on the horse before you use Quest. With Ivermectin, Fenbendazole and Pyrantel Pamoate, it's much less likely to overdose them. With Quest, you need to be sure to be accurate. I've used Quest ever since it came out and have never had a single horse have an issue.
After you've done your thorough deworming and he's gaining weight, I'd have an FEC done, and then only deworm after an FEC with whatever your vet says. If he's not exposed to a lot of other horses and his pasture isn't contaminated, then you'll probably get down to 1X/year deworming.