OP, yes, most families who accept an invitation for their child to join another family on holiday will send some money.....but the point is that you cannot know how much this will be or what it will cover, unless you make a specific request for a certain amount, whilst making the invitation. If you don't specify an amount, people might feel it is rude to ask and send their child with £10 to buy a round of ice creams, assuming you will eat in all the time, or £50 or £200 - there's just no knowing - and as the host, you would have to ensure the guest child was fed. So unless you specify that the invitation involves them paying for their meals, you cannot just assume they will know that is the expectation or have the right money to cover a totally unknown cost.
It's all about communication, not money here. The problem is that you haven't communicated what you want to happen - you seem to think it is obvious that the other family will expect to pay for his meals, but the multiple posts on here should give you a good indication, that many people won't expect this. How is this issue sorted - quite simply by communicating your expectations at the point of inviting a then the other family accept or decline based in full knowledge of the financial implication for them. Now, a short time before the holiday it's just not reasonable to say 'and you'll need £150 for food' - because that family may have declined your invitation if they had known £150 would be required....and now it's too late to decline.
So fine to ask for a co tribute on towards the flight or food or accommodation or days out......as long as you are clear when issuing the invitation. Lack of communication in these scenarios is the source of nightmares, misunderstandings, resentments etc. And if you haven't communicated at the point of invitation you'll have to expect to pay for most things in a worst case scenario if boy comes with very little money.
Given your lack of communication so far, once you're in the plane, I would have a chat with the boys about expectations and suss out how much the guest has with him. If he has plenty you might then be able to suggest that he gives you X towards food and then you'll pay for meals out and food in, or you could suggest he pays for his meals out but you will cover the food whilst in....but be clear. This is all rather late and you might find he's a bit surprised, but better to do it at the start of the holiday and not Spring a bill on him in a restuarant....then if he knows you want X he can budget with the rest of his money.
And again, for kids if this age who will be off doing their own thing much of the time, no one is saying you supply them with daily sprending money and n unlimited budget....but if they are with you at food time, it really would be normal to pay, and people don't tend to invite a guest if they aren't prepared to do that.