Do not try gerbil, or anything else for that matter that you cannot regularly get or afford to feed.
Thats the last thing that you want is your ball to ONLY eat gerbils......
Is he a hatchling, or is he older. If he is older, he may be going into the breeding mode. Some males do that.
I would suggest keep trying every week what he was eating before. Even if it is too small, give him 2.
Hope that helps!