If you already have him on FT, you might want to avoid trying live. Sometimes they don't want to go back to frozen after eating live.
Some of them just don't like rats. If you try switching him and he doesn't take the FT rats, you may want to try jumbo mice.
They're about 35g to 40g, similar to a weaned rat.
Adult males don't need a lot of food. Some of them only eat four or five times a year, so a jumbo a week should be fine and you'll still have plenty of weeks where they don't eat. If you're really worried about him getting enough to eat in a single meal, you can give him two jumbos.