Ok, my reply will be detailed, and logical as well.
Well you can trust him on the fact that he said he'll love you the same, however what about attraction? I mean there is a difference between love and attraction, hope you know that?
Yes, he said he'll love you the same as before but he didn't say that he would be attracted to you the same as her was before, so it's possible that he may no longer find you attractive because attractiveness is also important in relationships. Yes physical looks, attractiveness is indeed important, there is no denying it.
Hence both attractiveness and love are important in a relationship, because a relationship can also exist if there is no attractiveness or little attractiveness but love is there, then it will feel like something is missing, but the relationship will continue to exist because of love ( factor).
The best scenario is if both love and attraction are present equally
Coming back to the topic, he said he won't stop loving you but then may not find you that attractive as before. Hence that will be a risk you will be taking if you go for a short pixie cut hair.
I have a suggestion:
What I would suggest is, you should ask him upfront about the attractiveness factor, you should ask him
" If I get a pixie cut will you be attracted to me just like you were before?"
Now, if he says
" Yes, I will still find you attractive just like before and I will also love you the same way as before".
Be upfront and straightforward while you ask him this, don't ask him indirectly.
If his reply comes like this then you can be 100% sure and you can believe him and have your pixie cut.
That's all from me.
Please ask me if you didn't understand anything.