Hi,
So how big did you want the cake to be? Is it a one tier, three tiers?
If you were going to make a 3 tiered cake you could have each tier as a different theme, and each tier as a relating colour, for example,
Bottom tier: could be the gaming tier, could be dark grey, and decorated with fondant cutouts of xbox, playstation, the controllers, well known game characters etc.
Middle tier: could be dark blue, with another theme of your choice and decorated with objects from that them
Top Tier: could be dark green, this could be the football tier, decorated with cutouts of footballs, football shoes, footballers, goal posts etc and the topper to the cake could be a gumpaste/fondant or modelling choc figure of your son in football gear kicking a ball.
If you did a one tier cake you could have a full scene on top if the cake, your son sitting on a sofa, gaming, with a little fondant tv, playstation, and you could have the things he likes on the floor around the sofa, like footballs, his favourite foods, etc
Hope this helps, good luck! 
Thank you for that great suggestion. I didnt think about a couple of tiers but that would be a great idea 
Time might be against me now for that one but the cake with a settee is also a really good idea. I'v made a settee before and I know it's not that difficult to do.
Thanks again Jamandbuttercream. You have some really good ideas. Dont know what i'd do without your help. 
Happy to help!
I see a one tier cake with a settee, a boy sat on it holding the game controller, there could be a tv with the playstation/xbox underneath, you could even make a little image painted onto the tv! and he's surrounded by things he likes, footballs and you could even have things like tiny half eaten pizza slices, pizza boxes, cans/bottles of his favourite drink, favourite magazines and favourite dvds!
good luck! hope you upload a pic when you've made it!
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