He is blowing with a relaxed embouchre at the start of the glissando, fingering the starting note then gradually sliding the fingers off slowly but also slightly tightening up the embouchre at the same time as he slides off the keys going up to the end note of the glissando...
to do this try taking 2 or 3 notes, say, G A and B.
1...play G then slowly move up to A but relaxing the embouchre and slowly sliding off the G key up to the A whilst tightening up the embouchre at the same time slowly til you can merge the the 2 notes seamlessly, try this SLOW and QUIETER at 1st.
2...repeat this using notes A to B til you can do the same as in point 1 above
3...then try the same technique as above but going from G up to B
have fun
if you need more in depth then PM me and i will maybe skype you so you can see face to face or i can put a soundclip together that you can work from.