It is directed that way due to the impact of having food in your stomach. It requires an empty stomach for adequate absorption of the drug. Waiting the 30 mins - 1 hour means that there's enough time for the drug to begin being absorbed before the food dilutes this. It also requires waiting at least three to four hours before taking any supplements that contain iron or calcium, calcium-fortified juice, or cow’s milk, which can be difficult.
It is possible to take Synthroid at night time, there is some concern that there will still be food from dinner, although most patients haven't eaten a lot just before going to bed anyway.
Studies have been done that have shown an improvement in thyroid hormone levels if taken at bedtime compared to in the morning.The key with Synthroid is taking it regularly and consistently, if night time suits him better that is an okay option.
Bolk, Nienke MD; et. al. "Effects of Evening vs Morning Levothyroxine Intake: A Randomized Double-blind Crossover Trial." Archives of Internal Medicine. 2010;170(22):1996-2003.