Advice on getting dual citizenship

My husband’s Mom is an American citizen. She was born in the USA and lived there from birth until 13-14 yrs, she then moved to Canada, got married at 19 to a Canadian and she had her son in Canada in ‘82. She got divorced, moved back to the states in ‘88 and took her son with her and he lived in the states for a couple years. He went to 2 elementary school in that time. Class of 88-89, and class of 89-90. He then came back up to Canada to live with his Dad. She stayed behind and has resided in the states ever since. My husband wants to get his dual citizenship. Please advise best course of action. Thank you.