My 7 Yr Old Is Complaining About His Tooth Hurt

My 7 yr old is complaining about his tooth hurt that has a cap on it!! Well we took a look at it last night and we didn't see anything wrong with the cap or tooth...went to compare it to the other side and noticed a abscess on his lower gum right next to his tooth. He says it doesn't hurt and he didn't know it was there.. Have been giving him Motrin due to the other tooth hurting him. On our second day and the abscess seems to have gotten bigger..also tomorrow is Christmas!! Please advise