My husband and I are legally married and together, however we separated. We have a 3 year old. I had to go on a trip for a week and a half, so I left our child with my husband. I was suppose to pick him up when I got back. However, when I called my husbands mother { my only way of contact} she said they were gone and something about he went to the Dhs. Anyway, I don't know where they are, they won't tell me where my son is? Is it Illegal for him to not tell me or to keep my son from me? What can I do?

You go to Court and file for the return of your son. If you were married and there was no legal separation (which would spell out the rights of both of you) your husband had the same rights to your son that you had - and that includes taking him "somewhere."

I assume you have called the Police. What did they say?

I don't know what "legally married and together, however separated" means.

When did this happen?

While he has equal rights he doesn't have the right to hide the child from you. That can be parental kidnapping. You need to go to court immediately and file for return of the child. You will need to prove he was living with you primarily before you had to go away.

I have not been t the police yet because I am not sure if they can do anything. I don't know what to do first. This happened yesterday and we are legally married but we decided to separate just nothing on legal paper.

The police can't do anything, yet. At this point it is a civil matter. Until you get a court order awarding you custody, the police won't do anything.

Tuesday morning be in court

I suggested calling the Police so that there would be a record. Yes, I would go to Court AND get a divorce Attorney immediately.

... she said they were gone and something about he went to the Dhs. ...

Where, who, what is/are the "Dhs"?

DHS is usually Dept of Home Services or similar. It appears the husband's mother told the OP that the husband and child were gone.

And to be somewhat blunt, this is what happens, ( mother or father) in too many cases, because there was no legal and formal child custody paper work.

At this point either one of you have the same rights to keep the child, and there is no requirement that one has to give them back.

Thus time to get into court

The one fly in this ointment is that the father is hiding the child. Had he simply refused to allow the OP to take the child, he would have been completely within his rights. But, by hiding from the mother he is violating her rights and the courts will probably not like that,