It's only been 5 months. There was no way in hell I'd have wanted to fall pregnant when DD was that age. No way in hell.
She's 7 now though, and I'd quite like for her to have a sibling. Ideally, I'd have given her one when she was about 3 or 4.
Your son has a long time before he's going to care about siblings either way, and there's a big chance your mind will change over time, so don't stress about this kind of thing just yet. It's way too early to feel selfish or anything at this point.