He is following a group of people who say that you can not make sects,
That group is saying the truth. You can not make sects.
the people who call themselves debandi, sunni, ahleHadith, and others have divided Islam, and are not Muslims.
We can't make sure of this.
A Muslim is someone who obeys no one other than Allah and his rasul. A person may accept that his religion is nothing but the orders of Allah, and he lives according to the orders of Rasulullah, but he may also define himself as Sunni, Shiite, etc, even though he mustn't. This is the case of "assigning a wrong label to the right religion", and happens because of wrong education.
On the other hand, when the rules in Quran are contradicting with his fiqh, if he chooses the rules in his fiqh, he is not a Muslim even if he defines himself as a mere Muslim and does not add any different extra label to his religion (like: Sunni, Siite, etc).
I, on the other hand, think that calling them non-muslim is not right, and I am confused on this.
When you meet people who add labels like Shiite, Sunni, etc next to the name of their religion, ask them this question:
What do you do if the rules you heard from your ancestors (scholars, imams, madhabs, etc) contradict with Quran?
If he says he chooses Quran, then he is a Muslim. You can pray with him.
If he says something like "My ancestors are always right. They never contradict with Quran. You are misunderstanding the ayats in Quran. You are not capable of understanding the ayats correctly, or criticizing my ancestor because you don't know as much knowledge as they know.", then I suggest you don't pray with him, because he is probably a musrik.
How do I convince him?
When you come across with a different idea, you shouldn't ask the question "How do I make him think like me?". Rather than that, first try to understand what he is saying. You might be the wrong one. Or maybe he is indeed wrong, but you may realize a flaw in your opinions on that matter and improve yourself.