I agree with Little.
And, friends will always tell it how "they see it", not how it was meant from an individual person..
You asked him why and he stated that he wanted to buy you a ring but can not afford to do so.. But, hey..... he obviously thought about it and how to over come that....
To me a "Promise" ring says "your mine" and i love you...
If his finances don't allow him yet to buy you an engagement ring... chances in addition to what Little said, a wedding costs money to, children as well, a house...
He would be adding it all up... and one step at a time to achieve goals...
Don't give him a time limit... (don't keep secrets), talk to him without being up-set instead of your girlfriends. Only you know him inside out, they don't and can cloud your judgement.
Why not let him know that you understand all the costs involved in buying a house, a ring, marriage and discuss saving, a plan and tell him your understanding where time is concerened due to finances.
I bet you you will see such a relief on his face, that shows and he'll talk more about it all with you as a result.
And smile..

promise rings say "your mine"...
CW