Seems like he has a delay and when the sound plays it will last for the same amount of time he pressed the keys.
I know that in the Note-on page of a sound you can setup the delay, but it won't last the amount of time I pressed the key, actually if I stop pressing the key before the delay time the note won't sound at all
Presumably the Timbres used to try to recreate this sound are Programs in a Combi, suitably zoned across the keyboard.
For the Timbre that needs a delayed start, it might be worth copying it to an unused Program slot and editing this copy, trying different settings for the AMP EG. This edited copy will be the Program actually used in the Combi.
Try Start Level to 0, Attack Level to 0, and vary the Attack time to vary the delay. Set the Break Level (Dcy/Brk) to 99, to give the desired volume, and set the Dcy/Brk Curve and Time to get the desired swell characteristic to the sound. Set the Slp/Sus Level to 99, and the Release time and curve to suitable values, to allow the sound to continue after key off.
Note that the Time parameters in the AMP EG edit page are non-linearly related to the actual time. In the Parameter Guide, there is a table giving EG Time values and the actual times e.g. an EG Time value of 10 gives 10msec, 20 gives 44msec, 30 gives 104msec, and so on. I'm guessing that the Attack Time parameter setting will probably be around the 50 mark, representing an actual time of 464msec.
There's no guarantee that this will give exactly the desired result, but it might at least give some ideas.