There are obviously three different strands to this.
1 - Helping your 19 year old with insurance for his first car, taking him out to dinner, these could be perceived as entirely normal. In fact, I'd say entirely age appropriate. He's getting on with becoming an adult, but you're making the transition a little more pleasant, and doing loving things. Also maintaining your relationship.
2 - your boyfriend (according to the OP) is bristling every single time you do anything that relates to your child. He is passive aggressive, and referring to a better time, when you will be doing less with your child?! This is not ok. Why are you still with him? You've presumably managed having a child for the past 19 years, what has changed that you are now happy to have this person criticising your parenting?
3 - your subject line, and some comments, imply that you DO feel that somehow your relationship with your son is almost painfully special. Your title, presumably written without too much thought, uses very dramatic language. "Always" come first. You refer to him being an only child as if this makes it a more closely guarded relationship, and more precious.
It really isn't. You don't become more cavalier about your first child leaving home just because there's another still at home.
Maybe it's become an issue of money and time spent on your son, but maybe there's something in your approach to your son that makes your boyfriend feel a jealousy he didn't expect himself.
In the end it doesn't matter. You're happy, your son is happy, this man is being horrible to you both.