Sorry to hear about your loss bud, my sincerest condolences. As the Deachness says; the blues. It always did it for me, but then again I was raised on it. Start with something you're likely to know, like Clapton. It sounds counter-intuitive, but playing the blues out is actually often quite a therapeutic release. It's like you're both meditating and grieving through music. YMMV.
In fact, one of the great strengths of the uke is you can play practically anything and sooner or later it'll cheer you up. It's by its very nature a therapeutic instrument. I bet it'd take me a lot longer playing the blues on a guitar to get the same effect as a uke, so really the answer is: whatever you are happy playing. I'm sure that your teacher would have wanted you to be happy, and the uke is one of the best vehicles for that. So don't aim for one specific style or a set playlist, just play, and keep playing.
Last year I lost a very close relative and was basically inconsolable. Four hours straight, just me and my uke alone playing whatever (sitting on a comfy chair out in the garage) resulted in some sore fingertips and me being in a much better headspace. That's not to say you should play for four hours, just play. Are there any songs that remind you of this man? That might be a good starting point for the grieving process, and might help you to focus on celebrating your friendship.