Hi laraby,
While latex is the most durable of all the foam materials … all foam materials will soften and impress to some degree over time and softer layers will be more prone to this than firmer layers. With higher weight ranges it’s usually a good idea to choose a mattress with firmer layers because the layers in your mattress are already on the soft side for your husband’s weight and any foam softening can put you outside of the “range” where a mattress provides the comfort and support you need more quickly even though the latex itself hasn’t reached the end of its useful life and would still be fine for someone with a different body type or sleeping position (see post #2 here).
For those who are only comfortable with softer layers it can be a good idea to add them as a topper over a firmer mattress or to choose a component mattress where the individual layers are replaceable so that if the softer layers soften sooner than the others they can be replaced without having to replace the entire mattress.
There are some suggestions in post #2 here that may be helpful for a mattress that has developed soft spots or “virtual impressions”.
Zoning can also be useful for body types where the weight is more concentrated in certain parts of the body and there are some zoning suggestions (similar to gtwflorida’s suggestioins) in post #11 here but to do this inside the mattress layers (vs under the mattress) you would need to do mattress surgery and I would only consider this as a last option if there are no other options available to you because the layers in your mattress are glued and you would need to remove them very carefully with a sharp knife or utility blade which would void any warranty coverage that you have.
I doubt that PLB will custom build a mattress with a different design than their regular models for a warranty exchange and I think it’s much more likely that they would exchange it for one of their regular models.
Phoenix