K.M.
I would definitely return it. I know it costs $ to do so but it's better to pay the 15% now than to end up having to buy a new mattress in a few months because your husband is uncomfortable. When we bought our mattress we were told to find the one that feels soft enough (if you like a softer mattress) and then pick the mattress that's one level firmer. Over time your mattress will become softer and you'll end up getting the softer feel. Once you get the mattress make sure you rotate it on a regular basis. If you buy from the Original Mattress Factory you can sign up for their mattress rotation emails which are very helpful.
I am someone whose back hurts if I sleep on a mattress that's too soft so I can understand how your husband feels. I know you asked for ways to make your mattress firmer on one side but I really think you may end up regretting that. It's easier to make one side softer using egg crates etc... Good luck!