Hi Purple,
I think you really need to speak to your OH about this as it's obviously playing on your mind a lot, going by this and your previous post about the family holiday. It's clear that you think the financial situation is unfair and your DP should be paying more, but he may never even have considered that you might be unhappy with the way things are. The best thing is to just be honest with him about how you feel.
It's very common nowadays for people to just pay into a joint account to cover the household expenses rather than combining all income in one account. It does seem silly to be owing each other half for things all the time, why not suggest increasing the amount you both put into the joint account so that it would cover shopping etc?
As for the holiday situation, I do think it's unfair to expect him to pay half. My DP has 2 children and while I'd be more than happy to split all household costs equally with him there's no way I could afford to pay half of a holiday abroad for the four of us, and to be honest I'd be quite upset and annoyed if he was looking at holidays he couldn't afford without me paying for more than my share. If he thought he was going to struggle to afford what we were looking at, then we'd have to change our plans to something more affordable for both of us.
It might help to try and look at it from the other side of the coin? He probably is paying extra in small ways you might not even have thought of, for instance your mortgage is probably higher because you needed a bigger property with an extra room, your shopping bill is higher because of the extra person in the household, your fuel costs may be higher etc etc. They are small things but they do add up, and if your DP is paying 50/50 on all that then to me he's already doing his bit.
Also in your last post, you mentioned that your OH also pays for the board for the pets while you're away, which is £300+...perhaps he does that in recognition of the fact your paying for your son?
I did want to ask you though, is it really the finances that are the issue or is it that you feel you're not a family unit due to the divisions? I only ask because it seems like the underlying issue might be that you feel hurt that your DP isn't treating your son as his own (by making you pay separately for him), and if that's the case then that's the thing you should focus on when you speak to him as he may understand your position more that way. I know I'd be more receptive if my OH spoke to me in that way than if I thought he was asking me to pay more so that he could pay less himself, or have more left over for something else (savings).
Whatever the case, I think you should definitely talk to him about it, you never know, he might even be up for paying half :)