I do have burning on the tops of my thighs intermittently as part of hereditary neuropathy which has progressed up to my shoulders and hips. I pick my pants with care as some of them are more likely to cause trouble than others. A man would have far fewer choices, loose yoga pants are the most comfortable for me, they are a poly/Lycra /rayon blend. Jeans become unbearable after a short time, especially if I give into vanity and wear ones that fit closely as opposed to baggy, saggy ones. My favorite hiking pants are a nylon blend and they do irritate the skin, despite not being tight. I think the nylon fibers are the culprit- they seem to create uncomfortable friction.
A soft, all cotton khaki, such as L.L.Bean makes might be your best best, pre washed, and a size up. Polyester irritates my son, who has this disease as well, and wrinkle free finishes are chemically treated. If you tolerate microfiber, there are some extremely lightweight men's dress pants available. It is polyester, but has a softer hand.
If it is any consolation, this is a symptom that waxes and wanes for some of us, so it may improve.