I don't think the weight itself would be an issue for someone of his size, so it's probably just an unusual motion that his body will get used to.
At the beginning of his rehab, my son was doing one set of 20 lb curls, then quickly moved up to 2 sets. Then the weight was increased 5 lbs at a time to the 35lbs he is currently doing. His therapist told him that there wouldn't really be much benefit for him to go higher than that.
If you haven't done so, you should look into one of the specific strength/conditioning programs designed for pitchers. There are some exercises that pitchers need to do that you don't usually see in a strength program. There are a couple of programs that are advertisers here.