“Nothing wrong with keeping a civil or even friendly relationship but he’s over the line!”
Sums up the situation, but there are a few red flags in you post.
Joint account - who uses the account, both parties or one. If it’s one, then not a problem.
Borrowing money - huge red flag. Why does a grown man need to feel the need to borrow money of anyone - that tells me he’s not financially secure or savvy. How much does he borrow and how often? Is he good at paying it back? That alone would be a reason not to continue with this relationship.
Monthly Meal with son - not a problem.
wedding anniversary - weird to text Happy 30th when he has declared he wants to marry you. I guess it’s fine to mention in passing that they got married 30 years ago - you’re bound to reflect on the date, but weird to text ‘Happy…’.
Darling - I guessed he continued out of habit. Is he the type of bloke that calls everyone Darling. Have you told him that it feels weird he calls both her and you darling.
Referrals - if she has had recent work done on the area, eg car service done, fair enough. Not for totally random thing.
Taxes - that wouldn’t bother me.
Considering they split 15 years ago, it does feel a little entwined. Maybe she demanded things of him and threatened to withhold access to dc when younger, but he’s now 25!