D81 Form

I’m not sure if I’m in the correct category, but I received a D81 form from my ex husband, we divorced almost 2 years ago. I haven’t received any documents which state what I am signing and what the agreements are. He put his wages wrong on the form and has also stated that our son lives with both of us which he doesn’t, he lives with me. I have a few questions as i have no idea about these forms

Should there have been any more documents stating what i am agreeing to and what I’m actually signing?
Will it effect child maintenance? As it took two years for him to agree to pay for his son
Should i get my own solicitor?

He’s being very shady as he keeps saying he just wants me to sign it so he can send it off asap. He doesn’t want me to get my own solicitor “because they’re very expensive” and he’s said a few times that it wont effect CMS. He’s just saying it’s to agree that if either of us come into any money in the future then we can’t take it off each other. But i have no document to prove this is all it is.

Any advice is much appreciated as i have no clue what I’m doing! Thank you