He stands like that quite a bit and doctor’s have made educated guesses as to what it means. My bet is on a neurological condition.

https://vocal.media/theSwamp/why-does-donald-trump-lean-forward-a-closer-look-at-the-mystery-behind-his-posture

EDIT: Forgot to add, he’s now walking and swinging one leg out and then dragging it a bit. Apparently that’s another marker for a deteriorating mental condition.