I was initially going to enter "I will not forget him". Then I realized I was yet again falling foul of the vill/will false friend thing: "I don't want to forget him". And then I realized that förlora is "lose", not "forget": "I don't want to lose him". It was like a sad little story getting even sadder as I translated it!
Förlora is used for the abstract/psycholoigcal sense of 'to lose', as in losing a person emotionally, while tappa is more concrete, like dropping something from your hands. If the object of tappa is an abstract noun, it too can have an abstract meaning, as in tappa lusten 'lose one's energy'.
If you're his teacher: Jag vill inte underkänna honom. If you don't want to let him down: Jag vill inte svika honom.