Mary is 16 years old, that was usually considered old enough for monarchs to rule in their own right. But Mary will of cource seek Cathrine's advice so she still would be influential.
It would make him King Consort and his oldest surviving son King of England so marrying his cousin Mary would definately make sense. There were of course two factions at his court, the pro-French one around the deposed regent John Stewart, Duke of Albany, pushing for a French match and the pro-English one around his mother Mary Tudor, favouring an English marriage.