There could conceivably be problems re-entering the UK as you'll have to go to a passport desk as he's too young to do e-gates by himself as he's not with a parent ... Have all the evidence you can scrape together of parental permission, with maybe a photocopy of his birth certificate to demonstrate they actually are his parents just in case Border Control want to be sure it's not something shady. I direct personal experience of this - I took my niece and her child who has a different surname to Italy last year, there were no problems in entering or leaving Italy but she had to produce the child's birth certificate to prove the relationship at UK Immigration.