Sweetheart, for the same reason he is your boyfriend and not your husband. Fear of commitment. Get the test, try not to throw it in his face. He is being something of a jerk, but in this day and age of freely available sex, guys have the right to know for sure that the child they are going to have to support financially for the next 18-21 years (or more) and emotionally for the rest of their life, are from them.
I think you should put him out if he has cheated on you plus make him pay child support after the DNA testing is done
Sorry, didn't see your other post until RockRose mentioned it. If he's cheating, the reason he is asking you for a DNA test is that he thinks everyone behaves the way he behaves. Talk to a lawyer, get some child support.
I would say GREAT, do a DNA test, but let the court order it so he'll be on the hook to pay child support. I read your other post and apparently he is cheating. Since you have 6 daughters you are raising and now you're expecting a son, it's time to formalize his relationship to this child and get the financial support this child deserves.
Do the fathers of the 6 daughters pay child support? That would be something to pursue, too.