Guys if his knees are caving inward as he presses up, its more than likely not weak ADductors its weak ABductors. Do you ever do any glute activation? Your hips aren't firing correctly.
As a warmup do double/single legged hip extensions, monster/mini band side steps, Squats w/ mini band just above the knees (this will make the knees cave inward, automatically making you have to fire your hips to keep them out). If you don't have any form of bands then you can do side lying leg abductions (toe pointed down to the ground, heel up high) and side lying clams.
As far as your actual squat, do you try and "spread the floor" as you are squatting? If not, this is crucial, just like bending/pulling the bar apart on a bench press. Imagine you are on a long strip of paper in your squat stance, attempt to rip the paper in half under your feet. This action will fire the glutes and save your knees.
Edit:
Just read your post above about sitting all day long... thats a common issue causing weak inactive glutes. If I had to guess your hip flexors, primarily rectus femoris, are tight which will contribute to your glute weakness and inactivity. Foam roll or do some soft tissue work on your rectus femoris, then stretch it, then activate the glutes. For stretching it I would recommend a simple split stance hip flexor stretch (focus on squeezing the glute on the same side you are stretching, the knee thats on the floor) if thats not a strong enough stretch then elevate your rear foot either against the wall or on a bench and treat the stretch the same. Again in this stretch focus on squeezing the glute on the side you are stretching.
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