My [26F] life long best friend [25M] just made me an amazing offer. My boyfriend [25M, 6y together] doesn't want me to take it.

Hi everyone.
So basically, I had bulimia for 10 years and absolutely destroyed my teeth. I don't even have ANY teeth in the front. I have dentures which hide it but they are discolored, and painful and annoying. I have extreme self esteem issues when it comes to my teeth. I'll just be sitting there watching someone and notice how pretty their teeth are, and I feel so terrible and it's all I can think about. But I don't have the thousands to fix my whole mouth.
I have a male best friend since I was a baby. We are very close but have NEVER had feelings for each other. He knows my boyfriend well and they are practically best friends too. Well, he recently came into a LOT of money from an inheritance. The first thing he did was say to make an appointment with an oral surgeon, it's a gift from him.
I didn't want him to spend that much money on me, since it's such a huge gift to take. But I also felt hope and happiness that one day I could have pretty teeth to not be so insecure about. And the cost for fixing the teeth is nowhere near how much money he inherited.
My boyfriend and I have been together for 6 years. He loves me and treats me well. But he doesn't want me to accept this offer from my best friend. The worst part is he can't give me a clear reason why. He just keeps saying "it's too much money" and things like that, but I feel like he has reasons he's not saying. He's one to hide his true feelings a lot of the time.
How do I talk to him about this? Should I let my friend get my teeth fixed?
tl;dr friend offered to pay to fix my teeth but boyfriend doesn't want me to


Source.