In addition to the great responses here... um, just explain to him that his cousin and him are different.. you don't have to copy... that he is his own person.
when my girl was that age... she could understand when we explained things like this... ie: differences in people.
Plus, you are having a baby in 3 months. Maybe he is just regressing... regression in a child is a symptom of stress.
But be consistent.
I am sure he will go back to normalcy. But I think he is just still getting the hang of it... afterall, he had 5 days of doing it well... that isn't long. AND I'm sure he can feel the 'pressure' of everyone wanting him to just be done with it and be done with diapers already. And I'm sure he hears everyone 'discussing' it too, about him and his toilet skills. Any kid can feel the vibes and hear the parents talk.
when my girl was about that age and I had my second child... she went through a spurt where she had potty accidents too. But it passed. She was just adjusting to everything. Its a coping skill that for a child, this is what they do. They make mistakes, they have anxiety... and it gets manifested in their toileting.
All the best, he will be fine...
Susan