Ah that's a frustration when you go to the trouble of asking them to try things out and selecting what they want only to find your hard work need adjusting. He needs to remember that the foam will soften up a little over time too. Is it a wooden base you have? If so I find holes in the bottom do let air escape and enable to foam to compress a little easier. Sorry if that's not too much help.

At the beginning of every project , we tell customers that we make foams on the firm side to achieve a good finish and provide longevity due to softening. I think when you start the conversation this way , fussy customers are far more receptive. Your not pulling the wool over their eyes as it’s just a fact that the firmer the foam , the better the finish. If you look at modern seats , they are like concrete but are designed so well with regards to shape/lumbar etc that they are comfortable I have to admit that it’s hard to achieve Ergonomic comfort in a lot of classic Americans due to cabin size and layout, I guess it’s up to us to get this point across. We always push the point that the softer(more comfortable!) we make it, the less crisp it will look over a period of time . I think that’s a fair and genuine approach.
We’ve never done it ourselves but I’ve seen people cut holes out of the base and used webbing I guess to add softness/bounce . I would imagine that holes by themselves would only act like breathers ( which is no bad thing!) and aid the cover to return but without adding softness which you require.

I would remove the cover and foam, cut out the center section and replace with webbing. On his cost ofcourse. The little breather holes just keep the vinyl/leather from poofing when sat on until the air escapes.

Thanks for all the help. I tried cutting holes in some scrap foam and didn't like the outcome as I didn't notice much difference plus I felt like I was compromising the foam quality. I ended up taking the seat apart, and as it was laminated to begin with I took out some of the firmer foam and replaced it with a less dense foam. Also I did as Adam Gabriel mentioned, cut out the base under each side and replaced with webbing. All this was on his nickel as he had ok 'ed the original seat. He was happy and was alright with the extra cost. All is well that ends well....again thanks for all your input.