ilove - please don't put the maintenance into a shared pot.
You're able to support yourself and save at the moment. By moving in with someone, you should see living expenses go down as you're sharing living costs with someone. Therefore you should actually be in a position to save more money (or spend more money if you were the type of person who was a spender rather than a saver). Not be in a position where you actually have less money to save for your son! Your son is not costing you any more in living expenses by you moving in with someone.
And equally your DP is actually in a position of halving his living expenses too, so your son is not costing him. In fact you are subsidising his DC by the fact that out of 5 people, you contribute half (2.5 people) instead of 2/5 for 2 people. Your DP is contributing half (2.5 people) rather than for 3/5 for 3 people. With your new baby this arrangement seems OK. Even though I think your DP has an advantage here - you do more of the housework and he has someone to help with his DC.
The only way to even consider putting your son's maintenance money into the family pot is if the maintenance your DP pays for his DC goes into the family pot. A share is taken out for his DC (given to their mum) and s are is taken out for your DC (which you choose to save).
I am a single parent of DC. In no way would I want my DC to be disadvantaged because I choose to move in with a boyfriend. It is enough that I'd be expecting my DC to share their homes and lives with people that they have had no say in choosing. It's illogical that you can save an amount for your DC now but wouldn't be able to save to the same extent when you move in with a boyfriend
I would also want to keep the maintenace ring-fenced for future protection in case of illness, redundancy, etc as you have mentioned. I would not be asking my ex to set up a savings account, where I couldn;t use it if circumstances changed.
People talking about blended families etc on here - DC will not always be treated the same, they have different parents with different earning abilities, different saving/spending personalities. You can't make all things equal when there is such a wide mix of parents, new partners, step children, new babies involved. You can make how you treat them day to day (holidays, presents, love, attention) equal, which you are doing.