Guys are completely different creatures than girls. In his mind he is doing nothing wrong and he is doing things with his friends and having fun before the baby gets here. Lots of guys react to babies like this, not saying it's right at all, just saying that is how many guys react.
He didn't seem to react to the letter very well (he might have been overwhelmed) so maybe next time try and talk to him in person but, like another poster said, try to do it without acting like you are just being hormonal because guys ALWAYS blame things on hormones and then they don't take anything seriously.
Be careful though and don't seem too clingy. I remember in a past relationship I wanted him to spend more time with me and I kept asking him to and making him stay home to be with me and we kept getting in fights because he acted like he was a big martyr when he stayed home and his friends made fun of him for me "punishing" him.
It's like I wanted him to WANT to spend time with me without me asking and it hurt my feelings that he wenr and did something else. When I asked him to stay and hang out, he just thought I was being clingy and annoying.
That was a past relationship though (we broke up for completely unrelated reasons) and I am so lucky now because my OH spends every night with me and we go out to eat (even to Taco Bell, it's cheap and it's something to do). As far as him saying that taking you out for dinner is too expensive, what about the money he is spending on gas and all the other things for his bike? (And not to mention it seems weird to trade in a car for a bike that you can't use a carseat for right before a baby comes, but that is a completely separate topic)
I also agree with another poster that it seems like you need some friends to hang out with. It seems like you stopped hanging out with your friends when you got with your OH so maybe try to reconnect? If it is too late to reconnect with them maybe you can make some new friends. I know it is scary to, but with a new baby you have tons of excuses for finding new friends. After you have the baby maybe it will nice to take the baby to some playgroups and meet some other new mommies.
I am sorry you are going through this and you are not wrong for any feelings you have because they are your feelings. Hopefully your OH comes to his senses soon (he probably will, most guys eventually grow up) and realizes that it is only going to be the two of you for a few short weeks before the baby gets here so you should spend time with each other. Good luck!