How did you manage your money before ds?
£350 on stuff JUST for him that are luxuries to my mind is plain unacceptable! That's nearly my rent! £200 on Lego "just because" is ludicrous and bloody irresponsible frankly! Honestly I'd have asked made him to take both items back to the shops.
Is his self employed business actually viable? Yes it takes time to build a business but if he can't "get" the basics of running a HOUSEHOLD budget I dread to think wtf he's doing with a business one! I'd genuinely be worried he's the type to fuck up and come begging to you with a huge tax bill he didn't plan for!
Money is clearly tight right now you need to have the "needs vs wants" talk with him (geez is he 12? My dd knows this!) - and not just about money but about how much space HE gets in YOUR (plural plus kids) home! Am I wrong if I say I suspect he's selfish with his time too and you do the bulk of childcare and housework even when you're at work and even though 3 of the DC aren't biologically yours? Honestly what you describe DOESN'T sound like he is actually pulling his weight there either.
I suspect a lot of posts will suggest that you consider your and his income as one pool, deduct essential costs (and that INCLUDES ds's AND dsc's essential costs) that you agree on and split the remainder in half for each to spend as they wish. That seems fair BUT I would strongly suggest you DO NOT get a joint account with this guy. You can maybe have an account that's in your name only that you sort of operate as a joint account for direct debits and household bills?
Are you fully informed on his credit record?
However, gotta be honest I personally think paid "swimming lessons" are a waste of money at this age. There are lots of tutorials online for exactly the type of guidance you describe. I swam competitively at school age and before school I just had my dad teach me and he did a lot of the techniques that are in those tutorials, I'm late 40's this is not new knowledge Honestly if your dh feels the same then that's not an agreed cost and I'd say it should come out of your money BUT if you don't agree with frequent karting when there are cheaper options that needs to come out of HIS money. BUT that is for you both to discuss and neither should renege on a previous agreement by spending such large sums without discussing with the other
"it's important for development and confidence plus we swim as a family." Yes but you can do that at this age without a coach. At this point it's just getting used to being in the water, getting confident and all that requires is regular visits to pool not a coach.
Personally I consider the following essentials:
Rent/mortgage
Utilities bills
Food/groceries - so food but also cleaning products, necessary toiletries, replacement of household items eg crockery as needed.
Clothes & shoes
Childcare
Haircuts for older DC
Books - but no reason why you can't buy 2nd hand or use library where possible.
Toys
School equipment/stationery
Wifi - EVERYTHING is online now
Furnishings and decoration
Everything else is luxuries.
But that's just my opinion how I work my budget.
I'm thinking if he's a bit on the geeky side (as am I not a derogatory comment) get him on Martin Lewis's money saving expert site BUT direct him to the money saving tools and CHALLENGES. Unfortunately men are socialised to be goal orientated, so give him goals that SAVE money rather than spend it.
Normally I'd think it was off the ex expecting him to still pay maintenance - but I wonder if she's trying to recoup costs from when SHE had to keep bailing him out! Still not acceptably to use the DC for that though.
You know what? The more I read the more I think you need to say to him he WON'T be welcome at the new house unless gets his fucking act together!
Ffs! Why am I getting the feeling he pressured you to have DC to make it harder for you to leave?
"Plus a lot of abuse etc" from his ex? According to whom? Because to be honest in this scenario with this guy it makes me think of the phrase:
When you're accustomed to privilege equality can feel like oppression.
His claim of abuse could have simply been her saying "enough is enough"
"Private arrangment, I'm pushing hard for it to be agreed legally in court as it stops the nonsense" again PLEASE be aware you're only getting one side of the story - I think quite a few of us responding can WELL imagine a very different likely scenario here!
And the age of the kids, quick bit of maths... Child 3 was a baby when he split with ex. With 3 DC maybe she thought he'd FINALLY grow up, pull his weight and pay his way and when he didn't that was her breaking point.
He says he can't save - he could save £350 now! Just by returning his toys!
"Tbf he does also buy me nice things like flowers or wine as a suprise which is very lovely" except it's NOT lovely cos he's basically paying for those with YOUR money - you're buying them!
The more you post the more it looks like financial and emotional abuse to be perfectly honest.
At BEST even being generous he's easily described as a feckless father - yes even though he's paying maintenance - cos again I suspect you're frequently bailing him out!