If you both agree on what the current value of the property is (possibly doubtful), there's a shortfall of 89k that you are both responsible for.
In the initial funding of the decoration/etc, he contributed 60k (40 + 20) while you contributed 15k (10 + 5), so a difference there of 45k in the funds contributed to get the house to its current value.
If you both agree that the groceries and utilities elements of payments cancel out, I'd have thought that a figure of about 67k (half of the 134k, 89 + 45) would be what's required to walk away from it (though I can only imagine how much debate can/would arise over it). This assumes you both retain the existing loans you hold.
So it'd be the 20k (half of his original 40k), 17.5k in loans (half of the 35k loans total) and 44.5k (half of the negative equity of 89k) that you would be liable for based on the figures above (82k, so the 67k + the 15k existing loans you have).
This is based on the assumption that you agree/wish to have both made a 50/50 contribution to the entire funding of the house. In terms of his initial investment and the other costs paid to date, you/he may disagree on what was intended to be split 50/50 down the line... so something that will require a lot of discussion to find a workable solution.