So, your character is safe until he gets wet, then he feels the flames. Depending on the sprinkler system, the amount of alcohol and the extent of the flames you have a window of time when he is wet and before the flames are quenched. If his clothing catches fire before the sprinklers cut in then the severity goes up, and depending on how many layers of clothing, synthetic fibres can created a molten layer which can trap heat, themselves burn quite hot and take time not only to extinguish but also to cool the point they no longer do damage. If the fire catches him at ankle level early on, then there will be flame up around his legs and body, and his upper body may provide 'shelter' from the sprinklers for a while.
Realistically, you need to decide how much injury you want and adjust the severity of the fire and the timing on the sprinklers to suit. You also need to consider the long-term physical consequences - burns are remarkably painful, potentially slow to heal, and prone to infection unless treated carefully. Perhaps someone else on AW can give pointers on that - my own practical experience has been with relatively minor burns, and even those caused me problems for weeks.