3 mealworms a day will be enough to get him started. Many times pet store and wholesale geckos have not eaten in a very long time (if ever!) and will need to be started slowly.
You are on the right track...if he eats the 3 and digests them without problems then offer him 4 the next feeding. Keep offering him more until he is up to full meals (my hatchlings get 5/day and juvies get 10-15 every other day).
Your info regarding the temps. is not correct. They should be housed at 88-90 degrees at the warm end and 80-82 at the cool end. This will help him to properly digest his meals. They can ,and do, survive at lower temps but that's not ideal.
Good Luck!

John Yezbak
North Coast Geckos