Dependent gambling winnings $3000

My 22 year old son is a full time student and received a WG2 for gambling winnings of $2900. It says he has to file a return. The return asks if he can be claimed as a dependent on someone else’s taxes, which is yes. He enters are income information and now it says he owes $6000 on a $3000 gambling winnings. This does not make sense. When I remove him as a dependent on our tax, we owe $800 more. Please help! Can we still claim him as a dependent and can he check off no, if asked if anyone can claim him on their tax returns