Do not take this wrong, but 2 and a half years to potty train tells me, he was not ready when you began. You were ready, he was not. When a child is ready it only takes a couple of weeks.
Also, he spends so much time with all of you, he does not listen to you as a "Teacher" this is not unusual. Children know what they can get away with, especially family. They understand who can be worn down, who can be charmed.
Once children are in school, very precise rules and expectations are given on the first day and they are enforced each time. Children thrive with this. They like knowing what is expected and being recognized for their good behaviors. It is comforting to them to know that if they step out of bounds, someone will care enough to say something to them..
Instead of trying to teach him like in school, let him learn through the natural interactions of family life. Ask him for lots of help that take specific skills to accomplish.. Ask him lots of questions.
"Help me set the table. How many people sit at the table for dinner? We are having a salad, meatloaf, mashed potatoes and rolls. What do we need to set the table with? How many of each item. Please set the table for each person."
"I am going to make lemonade. I need this pitcher to be filled with water up to this line. Please find 8 lemons. Where is the sugar container. I need a 1 cup of sugar, which one is the 1 cup measuring cup? "
"Grandmother is going to the grocery store, help me write a list. We need milk. what letter does milk start with? What is the sound. What letter does it end with? What is the sound of that letter?"
"M looks like this, do you want to practice the letter M? What other things do we need at the store that start with the letter M??"
"Here is grandmothers grocery list, will you help her remember everything on the list? Can you help her find these things at the store?"
"Remember we always use an inside voice inside". "We do not run inside". "I like when you share." "My I borrow your butter knife?" "Thank you for sharing." "You sound frustrated, do you need a minute to feel better?" "Please you use your words" "I do not understand when you talk like a baby."
Whatever need to be corrected or encouraged can be handled like above. EVERYBODY in the house need to participate.. EVERY time..
I am sending you strength.. Hang in there.