Hi musmom8689,
If your mattress is too firm then there are some suggestions that can help soften up a new memory foam mattress in post #2 here (although dn has already covered some of them). It also includes a line to some topper guidelines that may be helpful.
If you end up needing to add a topper then you can always add a topper to your side only if your husbands side is already working well although this does mean that your mattress will have an uneven height from side to side. If you add a topper that covers both sides to keep the side to side height equal then I would use the thinnest topper that you think will give you the additional softness you need because adding too much thickness and softness on top could be fairly risky in terms of alignment for your husband (which could lead to the return of his back pain).
Phoenix