Hello, Thank you for asking your query at DoctorSpring.
The development of small bumps following a swim in a one year old, specifically points to Molluscum Contagiosum. It is a viral infection of skin that produces a rash that looks like small bumps. They do not produce any complications to be worried about.
In many cases, its left to go away on its own without treatment. It takes around 6-18 months for bumps to go awaycompletely. I advise you to show your son to a paediatrician once.
Hope this helps. Please feel free to ask follow-up questions