First, remember you're dealing with a puppy - nothing will happen overnight. It always takes time to form a behavior. DonC's advice about not making a fuss when you return is excellent. So is BrindleBasenji's advice to tell your pup you're leaving, he's staying, and all will be well - you'll be back soon. I can't say for sure that b's understand everything, but I do believe it really helps them when you talk to them. I always tell my b what I'm going to do and what I expect. Maybe it helps ME stay in a calm focused/assertive mind. Whatever, it works. Finally, RGK9Ruler reminds us of an important fact: a tired basenji is a good basenji.
Get your pup tired out. Don't make a big deal. Tell your pup you're leaving. Leave him with a treat that will occupy him for a while. Don't make a big deal about your return.
A second basenji might not be possible, but perhaps you could borrow a friend's dog for awhile (it doesn't have to be a basenji). It has to be the right dog - a calm, well-balanced, well-mannered dog who gets along reliably with your pup and who does not exhibit separation anxiety. If you could have this dog live with you for a couple of weeks or a month, or even off and on, it might do the trick.
I think these creatures are so interesting! They are programmed to manipulate us to get us to do what they want! Be strong and be patient.
Let us know how it's going.