Jewel
You sound quite young, I urge to to say no. There is no way your boyfy should even have asked you to do this for him IMO, especially as your relationship with him is so young
For a child to be left with a man he has only known for a month (am I right in thinking he has not seen this child because he was unaware he existed?), is both cruel and unfair on the child. The poor thing doesn't know either of you, and this would be totally unfiar on both him and you.
I implore you to talk to your boyfy about this and tell him you are not willing to do this. Maybe you are scared of him dumping you if you say no? If this is the case then he cares not for you.
To consider taking on a child when he is about to leave the country, is also unfair on the child and you and I think he has a damned cheek TBH.
I talk from experience like I said, of taking on 3 children as my own - I would NEVER, EVER do it again.
It will be the singularly most difficult thing you will ever do in your life. Not only will you be at school and work - and one of those will suffer, I guarantee that, you will have to arrange visits with mum, nursery, childcare and everything else - all without support form his dad because he will be 100's of miles away.
Can you speak to your mum about this? I think she may well advise you not to do it as well.
I wish you all the luck in the world, but you should seriously, seriously re-consider