I don't think BA's china is that special! I'd imagine a bit of supervision would be all that's needed, and in that respect, if you discuss this with the crew they will serve meals a bit out of sequence, to allow one of you to eat and the other to supervise. If he essentially eats what you eat, and you're ok with the menu (which will be in Manage My Booking) and can be pre-selected in advance, then that indeed may be an answer.
You can bring milk for, well, babies through security, see the Heathrow Airport rules here:
http://www.heathrow.com/more/help-wi...ty-and-baggage
You can also get milk quite readily in both Boots and WH Smith in T5, usually semi-skimmed unfortunately, though Boots (a) allow milk to be reserved in advance and (b) sometimes do sell ultra-pasteurized full milk.