I understand, I really do. I don't actually think your OP reads like you're trying to be mean or grabby - just that you're exhausted by the situation you live in and you want to make sure your own child is put first - rightly or wrongly, it isn't for me to say.
My situation isn't as complicated as yours, because there is just one ex and my husband and I have been together a very long time... but it has had it's fair share of discord and it can leave you feeling wild, unsure and determined to pull up the drawbridge.
If you haven't already done so, there is a book by Wednesday Martin called Stepmonster which delves into the feelings that surround sharing resources with your SC; who are essentially another woman's offspring, right there in your nest. It's really good and I would totally recommend it. (There is an audio version available on Spotify, but fair warning... you might find yourself shouting "I KNEW IT!" into thin air quite a lot!)
Of course, you asked about wills and I hope I'm not overstepping when I point out that it is clearly part of a wider issue that a book can help you address!
Regarding the money itself...
Give your son opportunities, teach him the value of hard work, make sure he has the tools to look after himself, and he won't "need" anything else from you. That is far more valuable than an extra 25% of anything. Give him the whole world and then let the inheritance be exactly what it is... "just money".
My immediately family have suffered through 2 x disputed wills after older relatives have died. The stress for those left behind far outweighs the value of the extra gold. So, do everyone a favour and get something fair and equitable hammered out in a lifetime interest trust will. They cost slightly more to create than a bog-standard joint or mirror will but the co-op are great and you can do it all from home over the phone / email. Once it's done, you won't really think of it again, but you will be grateful that you have something to protect you all.