Hi Dr,i Am Glory,my Son Is 12 Years Old,i Really

hi dr,i am glory,my son is 12 years old,i really wanted him to gain weight ,his very weak and skinny child he always been sick since he was little,he is fussy by eating food unless his favorite,,but veggies and fruits his not in to it,,i have given him different kind of multi vit,seems ok but nothing change,,dr,do you think i do need to deworm him ? and what best medicine to deworm for this age?