There are lots of modern equivalents of "Unguentum Merck" - emollients are the first thing most doctors will suggest. The ingredients of UM are "Cetostearyl alcohol, glyceryl monostearate, saturated neutral oil, liquid paraffin, white soft paraffin and colloidal anhydrous silica" - glyceryl monostearate is just purified coconut oil

Might work, but the back and the backs of the knees are not likely sites for dryness inducing irritation.

It isn't fair on a 92-year-old to try an endless string of things that might work after a month (but probably won't). Each of those months is a sizable chunk of the life he's got left. If it's bad enough that he's complaining about it (most eczema sufferers just put up with it) he needs it dealt with FAST.