What Causes A Lump On The Nose Of An Adolescent?

my 16 yr old son developed a bump on his nose in Decemeber of 2015. Many thought it to be a boil. I took him to his pediatrician, who prescribed and antibiotic. He had to take 2 rounds and didnt seem to help. Then in May of 2016, it went away. August it reappeared bigger and followed by a cluster of smaller ones. I then took him back to the doctor, who then gave him another antibiotic and retinol cream. This did nothing. I then took him to a dermatologist, who diagnosed him with acne, prescribed him a stronger antibiotic, told him to take the retinol cream, gave him a second cream of Onexton and told him to do warm salt water compresses. This was one month ago and nothing has changed.