Has he ever eaten food? was it pureed or finger food? when did he stop? Have you tried reverting back to his original foods?
I have a 4 year old who went off food about a year ago - i know there were changes to her lifestyle that coincided. We are making very slow process towards her eating more foods.
My tips are do not make a single bit of fuss about the lack of eating, do not try and persuade and do not discuss his lack of eating in front of him.
Every mealtime sit and eat what you give him. Ignore any behaviour from him. Put the food in front of him, eat yours, chat about your day, don't mention the food. When you are finished take his away also. If he has touched anything or eating anything give lots of praise. If not, just don't respond.
The fact that he likes weetabix would make me tempted to sometimes give it for a dinner also, alongside your usual meal. Have you tried that?
The messy play suggestions are a great idea also. Make them complete opposites to mealtimes though - e.g. sitting on floor, and not encouraging to eat, although you could eat so he sees and potentially copies. Baked beans are good fun!
I know its so worrying as parents to have children that don't eat, but so long as hes developing well, drinking and gaining weight, he will come out of the phase eventually!