Would a topper help my.husbands back pain?

We have a ne mattress purchased 9 months ago from be sons beds. It cost us over £1100 we laid in their gadget bed which looked at our sleeping position and weight distribution. We purchased a no.3 which is a medium soft bed. I am 9 1/2 stone and my partner is about 15 1/2 stone. My partner is now saying his back hurts in bed at night and thinks the bed is too soft for him. We have a solid divan Base for extra support. Would you recommend a topper and which one please? Our bed size is a super king.