DNA testing

My son's father recently told me that he did a DNA test on my son and found out that my son isn't his. My son will be 2 May 28th. Now he wants to take his name off my child's birth certificate but when we were in the hospital he gladly signed the papers even tho I told him while I was pregnant that if he had doubts for him to make sure he was there when my baby was born so we could do a dna test but he didn't he signed the papers. Is this test real?? He said he did the test because my kid doesn't look like him and because of something he said he remembered me saying 4 years ago. Any advice pls