Similar to @jca, I would recommend sending a text in the usual way (instead of an email). The time difference between Singapore and the East Coast of the US is 13 hours in wintertime (Standard Time), and 12 hours during Daylight Savings Time.
Here is a handy tool to coordinate “meeting times” (if you consider that a phone call is a type of meeting) between and among different time zones.
Other than that, the US phone number should work for voice calls, but depending on the carrier and contract, additional charges to the call recipient may apply. That’s another reason to suggest the text: give the other person a reason for why he or she may want to make the connection, and the option to decline if the timing or the reason for the call are not compelling enough for him to accept it right now. (And hopefully if he’s sleeping the text won’t wake him, but that’s on him to manage his phone notifications, etc.)