Last day, he showed me a ring online [link removed by Moderator] and told that he ordered it for me
What is going on here if you take a step back and pretend it is not happening to you personally?
Basically there's this person who is bothering his ex with unasked for behavior. Calling all the time, big surprises, asking for dates and now trying to propose with a spendy ring. That is not appropriate. Sounds kinda desperate/crazy/stalker-y sounding. They are
exes. Why is he harassing the ex like that? What is he after and what does he want to gain?
Don't let your soft feelings for this person cloud your judgement. This is NOT the way to go about getting back together.
This super speedy/overinundating thing might be great for HIM... but use caution. It might not great for
you. Where's the fire?
If you don't want him bothering you at all, tell him so, then block the number and don't answer the phone.
If you are considering dating him again, tell him to cool his jets and stop coming on so strong because it is a turn off. Give you some time to think about it without any more contact. You will contact him in ___ weeks. If he cannot respect your simple boundaries and keeps pestering? Tell him you thought it over and decided NO.
The hard thing about being poly knowing when self preservation /sanity "trumps" our need to be compassionate for others in or lives.
I think one's own self preservation and sanity always comes first. I think one
has to put their own well being first. Not in a selfish "memememe!" way, but in a "I have to put my own oxygen mask on first before I can help others well" way.
Otherwise it is like saying "I put everyone else ahead of me even if it kills my own self preservation and my own sanity." Who wants to live like that?

It is not self respecting behavior to do self neglect to the extreme.
If you help others before helping yourself? Then you are not operating from a full tank of gas in the first place. You risk burning out, spread yourself too thin, confusion, stress, etc. And you don't give the best help you could give running on low gas anyway.
Could learn to fill own gas tank first.
Galagirl