Please don't make any 'threats' about seeking legal advice or a share of the house. Don't put him on red alert any more than he already is.
Go stealth. Do seek legal advice, but do it quietly. As others have mentioned, legal representation can be expensive, but there are some solicitors who offer free 1/2 hr consults. And I've heard that the CAB can be helpful. If you can't get free advice, then you'll have to balance the cost of a solicitor against what your shitty P is willing to pay you and the cost of replacing DS's things.
I'm assuming 'DP' means that you aren't married so if you do see a solicitor what needs to be discussed is child maintenance and the 'removal' of DS's 'belongings'. Go ahead and ask about any financial interest in the house, but I doubt there will be. Listen to what the solicitor says and then make whatever plans need to be made.
I expect there won't be any way to get an order to remove what you want to take if DP refuses to 'let you', unless you can prove that you've paid for them. This is the crux of what you need to settle with the solicitor. If not-so-DP paid for the nursery furnishings, clothes, etc and can prove it then you may be SOL. Just as if you paid for them (and can prove it), the furnishings are yours to take. Legally, they are not DS's. They belong to the parent who paid for them and can prove it.
Your choice then (or if you cannot get legal advice) would be to either take what money is offered or to take them when he's not home. And you'd have to balance the latter against the likelihood that he'll try to get them back by accusing you of theft.
My BFF did a 'mid-day flit' when she left her abusive exDH. She took exactly half of the household furnishings plus all of her DS's furniture/belongings. He threatened shit but never did anything. I think a lot of bullies are like that. But they were married so it was all up to a judge to rule as to who got what as it was all community property. I think it's not so clear cut for unmarried partners as legally everything not shown as joint property is 'yours or mine' as opposed to 'ours'.