I went through some trouble and put my son in the care of his paternal grandma until I got back on my feet and healthy again, I am now asking to have him back and she is refusing, he has been in her care for 1 year.

I work for social services.
Providing you can prove that you are able to look after your son and can provide a secure base a loving caring environment with food and warmth.
That you can give him stability .
You should be able to apply to have him back .
Contact your social worker and your solicitor to make application to the court for him to be returned to your care.
I presume you did not give up your parental right and that you did not sign a guardianship paper for them to have him .
I am presuming that they took him on as "Family and friends "
Good luck
But for your son's sake I hope you have turned your life around as the court will not look on it kindly if you mess this child about .

First question is whether this was a formal or informal arrangement.

Yes before we can answer this question accurately, we need to know under what conditions you left your son with his grandmother. If this was just an informal arrangement you should be able to just take him back, if not you have to go to court. Any question on law needs to include your general locale as laws vary by area.

Aside to epm:
Welcome to AMHD. Thank you for trying to help our members. But, as Joy and I said, you are making assumptions about this situation then may exist.