I've never raised a little one from less than a year old, so I hope that others can give you a better idea of how much he should be eating and/or how fast he should be growing at his age. But one thing I can say is you're lucky he already likes veggies.
Otherwise, all I can say is don't try to push him to eat more than he wants and don't stress out over it as long as he's still growing fairly steadily and not losing weight. You could also try some other types of insects (if you can get them) such as roaches or silkworms (for example) to see if there's anything he likes better.
Instead of telling me how many bugs my beardie should be eating per meal in terms of numbers, my reptile vet makes a little circle with her fingers over his body and tells me this is about where his stomach is and about how big it is, so I have to imagine how many chewed up crickets it would take to make a wad about that big.
Better UV lighting should help. Otherwise, I hope others will chime in about the best feeding schedule for a 5 month old. :?