My baby is 4 months old now and he also has this rashes at his neck. I think it's due to the sweat.
Now I am putting Vaseline on it and looks like it's working so far. I only use it for a few days.
But I also think it's better that you can use a wet cotton piece to clean his neck during the day from time to time to keep it dry and clean. It can help to prevent it.
My son had the same problem when he was new born. I had visited specialist and GP but in fact no particular concern and just gave me some liquid to spread over the neck area.
In my exp, when he got sweat, milk remains...in the neck, the red spots would come. So, do remember to clean and wrap it more often to keep it clean.
My son's skin is super sensitive. Easily get red, itchy. One time, he worn a plastic bib...his neck area immediately got red and itchy seriously. But then it would return to norma after cleaning it with water!
So, watch out for every stuff you use for your BB!