Quads & hamstrings constantly co-activating, extremely painful

My (otherwise healthy) adult son has undiagnosed chronic pain in his legs. He has seen a neurologist (MRI, xrays, etc) and a rheumatologist (blood panels, needle EMG) --- all normal. My son believes that the pan is caused by misbehaving muscles (constant co-activation), but no one can determine why they're misbehaving and fix the problem.

Let me start with the pain: The pain is in the front and back of both thighs, the quads (he points to rectus femoris and sartorius) and hamstrings. It's a dull but strong ache, like a muscle ache, not sharp, not spasms or cramps. It doesn't feel like "nerve" pain, rather muscle pain. It's equal in both legs both in intensity and location, but roughly 60% quads vs 40% hamstrings. There is no tenderness or weakness, and it does not affect his mobility. It is steady and constant; doesn't come in spasms, and is present day and night. On a scale of 0-10, it is 5-7 at any point in time. It was not caused by an injury; it just appeared years ago, progressed and seems to have plateaued. When it first appeared, he was able to control the pain with OTC meds; now he's been on opioids for several years. There's no pain anywhere else in his body, including pelvis, lower back or lower legs. Pain can be somewhat relieved by stretching or walking around, but pain control is not complete and is temporary

Now for what he believes is the culprit: His quads and hamstrings are constantly tensed, whether he's sitting, standing or sleeping. He's unable to relax them, and describes it as doing never ending leg-lifts. All remedies to reduce the tension (i.e., prescription muscle relaxers, massage, walking, stretching) are only partially effective and of course temporary.

Side observations: I believe he has anterior pelvic tilt, and he's always had a peculiar gait and stance. He walks noticeably tipped forward, and rather than a smooth stride, he generally takes relatively short, springy steps --- often with a near crouch, and arms slightly out from his body. He also typically stands tipped forward, not erect, knees slightly bent, again as if in a crouch..

So the question is: what's could be causing the quads and hamstrings --- which are antagonistic muscles to boot --- to be constantly co-activating?

Thank you for any insights or guidance.

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