First, don't forcefeed him. Second, leave him alone. Third, offer him food.
You have had this baby for only one day. He has been carted around and his world has been shaken. Of course he is stressed. He needs time to readjust. Forcefeeding him will be traumatic and so would messing with him.
Offer him crickets and then leave him alone. If your temperatures are decent and the enclosure is adequate, in all likelihood he will start eating on his own. They can go days, even over a week, without eating and not suffer any problems.
You mentioned he is already thin. If the previous owner was forcefeeding him and harassing him, then it is all the more important that you not continue that process. Give this little guy a chance to do things on his own. Your only responsibility at the moment is to ensure he has good temperatures, good enclosure, good water, good food, and a peaceful home. Give him those and he will probably thrive.
Salvators
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^x^ Bloodbat ^x^