Why does my partner always go out after work?

I hope this doesn't sound too petty, but I am getting really upset about how much my partner goes out after work during the week. We have a 5 month old son, and during the pregnancy he was fantastically attentive. I should also mention that at this point he was unemployed while I worked. When our son was 6 weeks old he got a job which was fantastic news as he was really starting to get low about lack of employment. But since then, I feel that I barely see him during the week. His hours finish at 5pm but sometimes he has to stay later to finish a task before he can come home. I understand this totally. But if he does finish on time, he will always go out for drinks with people from work. In the end he ends up coming home on time one week night out of five. When he is late, our son has gone to sleep so he doesn't get to see his dad and I end up going to bed on my ow (when he is late he gets home at 10). I understand that it has not been long since he started the job and he wants to get to know the people he is working with, but they are single people who have no one to go home for. We have spoken about it and he has apologised for not prioritising himself properly and putting his family first and promised to improve. I think going out once a week with friends or colleagues is reasonable, especially as there are other nights in the week when he does have to stay late to do actual work. But since we have spoken things have not improved. He now rings me in the day to ask my permission to go out. I feel obligated to say yes as I don't want to tell him he can't have a nice time, but I feel resentful towards him as I have to stay in the look after the baby and spend the evening on my own once he has gone to sleep. I'm going back to work full time in a month and I am worried that he will continue to not be around, and I will be left to work and sort our son out with no help from him. I don't know if I can do it. He has reured me that he will get himself into gear when I do go back to work in all aspects (housework as well), but I am doubtful as he has ured me about other things which have not happened. I just don't know what to do and say to him. I have told him that I feel neglected and hurt, but he is still acting selfishly. What I hate most of all is that he always stays out late on a friday night, last week he got in at 4am and the friday before was 2am. He then has a hangover on the saturday so isn't any use helping to care for our son, which is a real shame as he is fantastic with him when he is around and not feeling the after affects. Does anyone have any useful advice about this situation? Should I talk to him again? Will he get his act together when I go back to work and help me with our son? I just feel so frustrated and upset about the situation.