Your purchaser will want to see bills/receipts to show that you are up to date with utilities, IBI, basura and community charges for the apartment. The liability attaches to the property on these items so if he lets you get away with debts, he will become liable. I am not sure that a purchaser is going to be too worried about non-resident income tax which is personal and stays with your son. As this tax is paid in arrears, a diligent purchaser may ask for evidence that you are up to date but may also want a retention for the unbilled amount. The seller is also liable for plusvalia - a town hall tax on value added. The town hall can tell you what this will be but it's normally not more than a couple of hundred euros.
The purchaser is responsible for paying the 3% (of the escritura price) capital gains tax retention into the Hacienda. The notary will not sign of the escritura until he sees the receipt from the bank saying that it has been paid. Some estate agents ask the seller to pay it on the day of the notary but personally I don't like that as, if the purchaser drops out at the last minute, you will have the devil of a job getting the money back from the Hacienda. Given the drop in value, your actual liability is going to be zero so it's up to you to claim it back. That's when the Hacienda will want to see you up to date with non-resident income tax or they will add interest and fines before repayment.
If you agree a sale, make sure that it is agreed that the purchaser pays the notary and property registry fees. That is the standard practice but it can be varied by agreement, so make sure it is clear at the time of agreeing the sale.