Do you thinki can improve my weight?

whenever i eat something, it doesnt stick to me. i can eat two large pizzas without being full and still wont put on weight. is there a way i can? im 18. im 140 and im 5'10. im slim... maybe i can talk to a doctor? i need to put on ten pounds so i can wear my speedos to the beach comfortably. i already have an athletic body