Should be feeding for around 15 minutes 2-3 times a week. These guys don't overeat, so he will stop when he's full. I personally use calcium every feeding and multi-vit once a week. Always allow a day inbetween supplements.
Keep feeding crickets, but don't give up on nightcrawlers. I used to feed crickets & locusts mainly but have noticed since I switched to nightcrawlers as a staple, they starting eating much more in quantity and pooping seems to be easyer for them.
I have a simple system, I rinse the worm in dechlorinated water, I dry it off with my towel and then toss it in a tiny container with a tiny amount of supplement, shake and feed.
I had a few who refused them too in the beginning, but now all 6 of my frogs happily eat nightcrawlers and even seem to preffer them.