It's clear he either doesn't understand mortgages himself or is trying to make sure you don't.
You bought the house for a certain amount.
Mortgage was £245k
There must have been a cash deposit as mortgage isn't for 100% of cost of house, it would have been 95% max, or 90% or less.
You pay off the mortgage, the money borrowed from the bank. So that at the end of the term you owe the bank £0 and the whole house is yours.
The mortgage payments each month pay off interest and some of the capital (the mortgage money you borrowed). At the start of when you borrowed, say it would be something like 98% interest and 2% capital. Your monthly payments will stay the same but the proportion that goes to pay off interest and proportion that pays off capital will change.
By the end of 15 years your payments (assuming no extra money is borrowed) will be maybe 70% interest and 30% capital, so the longer you have the mortgage the more capital you pay off each month, so that by the end of the term, your last few payments are nearly all just chunks off the capital.
What your husband has said makes no sense.
Plus the capital, the amount you owe the bank, should have reduced a lot more. If you've only got 8 years left of the mortgage you should have a significant amount less owing.
Anyway. Get the facts from the paperwork, not your husband.