Dr. Rishimba please advise on baby rash

Dear Rishimba
My son is 10 months old now. Here is the link of my previous postings regarding him, if you think it may be helpful:
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Since 10 days ago his bowel movements has changed. He gets constipated and he has to push a lot to do his bowel movement. First part of his stool is almost black (not tar black) and like small hard balls. The rest of the stool is dark and not as hard as the first part. The stool is very very dry. I have changed his diet to more fiber but still the same type of stool. Now since 3 days ago some red spots appear on his torso, front and back. They are less visible in the morning after he wakes up. They are really small like red dots and some of them have a tiny white head. They increase during day to the maximum in the evening. He doesn’t have any problem with them, no itching, and no fever. He still has a runny nose from the cold he caught about a week ago. Doctor diagnosed these spots as rash.
What do you advise me to give him?
By the way his constitutional is phosphorous.
Thank you