Verified answer

It sounds like a lovely gesture on your part and I am sure she would appreciate it. I'd run it past her, more so you dont buy something she may have bought herself or another family member has

I think that's very nice. It would be especially good if the present were related to something the two of you talked about or did together. If the child is too young for that, just make it something small that you think he would enjoy.

It's a nice thought, but if you're in doubt, ask your GF. She is the parent and what she says goes.

No one here has any clue how old her son is, how involved she wants you to be, etc. We are not clairvoyant.