I haven't read anything about fire skinks needing fruits or veggies to complete their diet. Aside from that, variety is something you can offer now and again, but as long as he is eating (dusted) prey items I would not be too concerned.

In any case, switching foods can be difficult sometimes so be patient. This is a great way for you to spend time with your skink, offering him interesting prey items while he glares at you

Hope this helps!