Dialing his number the way you do when he is in the US is the way to go. I can’t tell you why the call triggers his voicemail – either his phone is turned off or he is not in a coverage area.
One other thing: are you sure that he called home using the US phone with the US SIM card? Isn’t it possible that he bought a prepaid SIM card over in Uganda and used it to call home? In this case you would need to know his Uganda number and dial 011 256…