Greetings,
Edinburgh is a nice option to visit with son and hubby. Stating down some attractions which you and your kid can enjoy:
Gilmerton Cove
Auld Reekie Tours - Terror Tour
National Museum of Scotland - Fascinating Mummies
The Cadies & Witchery Tours - Murder & Mystery Tour
National Museum of Scotland - Catherine The Great
Greenyonder Tours - Hidden Gardens of the Royal Mile
The Edinburgh Dungeon
Edinburgh Zoo
The Queen's Gallery (Palace of Holyroodhouse) The Queen: 60 Photographs for 60 Years
Also I checked through
http://www.southalltravel.co.uk for your flight tickets for 3 people that will cost you around £261.18.
Happy Birthday to your son in advance. Enjoy your holidays.