IT's great you have done so well with this dragon. To get him to put on good weight, I suggeset sticking with the silkworms. These are one of the healthiest feeder insects out there. Mealworms are very heavy in chitin which could be why he throws them up. Adult crickets have hard exoskeletons as well. Try younger crickets, maybe about half inch long instead. Offer only a few per day, then offer silkworms and lots of fresh mixed greens. Maybe you fed him too much crickets and mealworms at once, as you didn't indicate how many of each you fed.
Make sure he stays well hydrated, silkworms help with this as well. You can also add superworms (bigger than mealworms, though rather fatty, but not as bad as waxworms). Being fully adult, the bulk of his diet should still be greens. Fruits and grains could help put weight on as well as crickets. Add a bit of cooked barley to his salad. Or grind up dry barly and spinkle it ontop of his salad. Other grains can be used too but shouldn't be more than 5% of his daily intake of food.
It may take awhile to put the weight on the dragon. Being older, they don't tend to eat as much as they did when younger. Have food available at all times, offer a variety and keeping him well hydrated and stress free will get that weight back on. You don't want to get him fat too fast, this may put more stress on his body than not having alot of stored fat. However I would try and keep him from deciding to burmate, since he doesn't have any stored fats. Though burmation doesn't cause much loss of weight, if he is still under weight, it could be very hard, even dangerous for him.
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PHLdyPayne