I agree with you two. I don’t think Alfie is the real father. A comment on another site suggested that the real dad is a grownup who doesn’t want to get in whatever trouble men get into in Britain for sleeping with a 15-year-old.
If Alfie is the real father, I blame the girl. Two years is a big difference in age at 13-15. Perhaps she wanted a baby and grabbed a boy not likely to cause her too much trouble afterwards. (But I don’t think Alfie is the father.)
I suspect the real dad is a married adult, perhaps even a relative.