The fairest way is that you get the percentage of the current value that you put in as a deposit, then any equity is split equally between you so, say your original deposit was 10% of the purchase price:
(current value - current amount outstanding on mortgage) - (10% of current value)/2
House prices have gone up a lot in the past 8 years so although you won't have paid off much of the mortgage yet (because you pay off the interest first), if the value has increased a lot, you might find that you owe him more than his mortgage contributions.
The average house price in 2013 was £176k, currently it is £277k.
So using this illustration from Which? (to save me doing the maths
)
If you bought the property for £222k 8 years ago (10% deposit of £22k = £200k mortgage), based on the increase in the average house price, it would be worth £348.5k.
At the end of year 8 (after all mortgage payments for the year have been made), you would still owe £151,414.
So the equity is:
£348,500 - £151,414 = £197,086
Your protected deposit of 10% is now worth £34,850.
So you then each get 50% of the remaining equity:
£197,086 - £34,850 = £162, 236
So the £197k equity is split approx £81k to him and £116k to you.
Based on that scenario, with a monthly mortgage payment of £948, you have paid £91k in total for the mortgage in 8 years. If he paid 50%, he paid £45.5k.
So he would only get £45.5k back if you return half of the mortgage payments, as he is asking. However, half of his (rightful) share of the equity would be almost twice that, £81k.
Obviously, it depends on how big your deposit was, how much house prices have increased where you live, the interest rate and term of your mortgage etc. I would do the maths with the real figures. It is likely you find you owe him far more than his mortgage contributions unless you had a huge deposit or property prices haven't risen much where you live.