Yes it would be safe, as a TREAT only.. But I wouldnt stop there, variety is an important part of husbandry. If you go overboard on the mealworms he may go on a hungerstrike and stop eating the crickets. Try many things, other than mealworms. There are a variety of roaches out there, mantid, moths, horns, silks.. the list can go on
i try to make sure on one bug is the main food. this way, they never go ona hunger strike.
you can feed all bugs, within moderation, and the appropriate size.