The son, a man in his 20s, needed hospital treatment to treat a head injury after being kicked repeatedly and losing consciousness in the attack at Wakefield Road retail park, Huddersfield.
The group of attackers fled the scene and police have now issued CCTV images of a man they would like to speak to about the "extremely serious assault" at 8pm on Monday.
West Yorkshire Police say the son was with his father when a group of youths were acting in an antisocial manner.Staff ejected the group from the restaurant but they remained outside the premises and were reported to be making gestures at the victim and his son.The victim approached the main suspect in the group outside the front doors to ask him to stop. The father, who is in his 50s, followed his son outside.One youth then confronted him and punched him to the face, causing him to fall to the ground.His son was then assaulted by the group, who kicked him repeatedly and caused him to lose consciousness. Pc Tracy Boucher of Kirklees CID, said: “This was an extremely serious assault.“I would also ask for anyone who recognises the male pictured to contact officers as we believe he may have information valuable to the investigation.“I would further appeal to anyone who witnessed this incident on Monday evening at McDonalds or who saw the group hanging around outside, to contact police.”Anyone with information is asked to contact Kirklees CID on 101 quoting crime reference 13170432380.Information can also be passed anonymously to independent charity Crimestoppers on 0800 555 111.