Ok so my girlfriend and I have a one year old son and my girl friend has some question as to who the father might acctually be. I am the father listed on the birth certificate. My GF has given guardianship of my son to her mother so she can get on her feet, but I would like to have him with me. My question is if they can order a blood test even if my GF and I agree that I am the father? No one else has stepped up as father of my son and I have questions about a blood test coming back conclusive that I am the true biological father.

If you are listed on the birth certificate you have certain legal rights, after a year, in some states ( if in the US) you will have more than in other states as to a challenge.

But of course the entire issue of her signing guardianship over without you signing it, may not even be legal,
Now can the grandparents fight it in court, of course, can they ask for a DNA test, yes they can, does the court have to allow it, no, will they, maybe. You just won't know till you try.

Chuck gave you great advice. Bottomline is you need an attorney to protect your rights. Did you ever go to court to establish custody, child support, visitation? If not, that was a mistake. The mom can't sign over guardianship without the consent of both parents. So I doubt if that was done officially. But since the grandparents have physcial custody for now, you need to go through the courts to have any change.