Small bumps around the neck at that age could be a few different things, like heat rash, eczema, or yes, possibly an allergic reaction, so it is hard to say for sure without a proper look. Each one has a different way to manage it, so I would not want to just guess and try random remedies. The safest thing is to bring him to your PD, who can tell you exactly what it is and whether he needs any cream or medication. Better to get it checked early so it does not spread or worsen lah.