A hot water bottle wrapped in a towel would keep him warmerwormst not so hot that it would burn him. They don't know enough to move off something that's too hot.
You said..."Add calcium powder to the crickets? and feed him less meal worms"...yes...you need to use a phosphorous-free calcium powder at most feedings on the insects. You should use a phosphorous-free calcium/D3 powder twice a month and a vitamin powder twice a month too. It's also important to feed/gutload the insects properly with dandelion greens, kale, collards, endive, escarole, squash, carrots, zucchini, sweet red pepper, sweet potato, etc. You can use a bit of fruit such as apples, pears, melon, berries too.
Proper basking temperatures allow for proper digestion so that plays a part too.