You have 8 month old twins, you've only been split from your husband for 5 months, and barely 3 months after that you started dating someone else who you've already introduced to your kids? WTF is WRONG with you?? There's been no time to resolve anything, and you can't possibly even be legally divorced yet! No wonder your 5 year old is messed up, because his parents have turned his life upside-down in a matter of months, and his mother is now being unbelievably selfish, putting her own wants ahead of the needs of her own children.
BTW, I'd say the exact same thing is his dad was dating someone else. You should finalize your divorce before you even *think* about getting into another relationship, and the man you're "sort of dating" has no business being around your children so soon.