waxworms are high in fat and there is no way to gut load them. If I recall correctly, they only really eat a mixture of grains and honey. Dusting them is about the only way to ensure your dragon is getting all the minerals he needs.
However, I don't recommend using waxworms as a staple diet, even for a short period of time for several reasons. They are high in fat, almost like 'candy' is to us. Many times dragons who get a lot of wax worms tend to refuse anything else ,so it will be much harder to get him back on crickets. And lastly, I consider feeding what comes out to being 'junk food' as a means to get your dragon up in weight, as being very unhealthy for your dragon.
A dragon that is under weight should be fed a completely healthy and varied diet, just offered food more often than normal for that dragon's age. This will allow them to put on a healthy weight, instead of a 'fatty' weight. Feed your dragon a variety of greens and insects and he will gain weight over time. It may take a couple months to get him up to optimal weight but it is better to do it the slow way than fill him mostly with waxworms, instead of more healthy insects.
I don't recall if you mentioned what age your dragon is, if he's an adult or still fairly young. Will assume hes at least a year old. I suggest feeding him a mixed salad with some insects sprinkled on top. A few waxworms added in once or twice a week is fine, and i mean only 1-3 served 2-3 times a week, then cutting them out completely and offered as a rare treat after a month or two has gone by. Instead of waxworms, offer more healthier insects such as properly gut loaded crickets, superworms (not too many of these though, 2-6 a couple times a week is good), silkworms, pheonix worms (these are a little small for adults, but a great insect to add in, as they are very healthy, high in calcium, lively and their fat ratio is much less than in waxworms which are a little bigger), butterworms, and many varieties of roaches, (young of larger roaches, newly molted adults of the smaller species).
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PHLdyPayne