a. Regarding the tense; are you trying to make it into the past tense? Or does he still wish to be a singer? Depending on the intention, the word ending will change.
b. No, you didn't use たがっている correctly. Should be an easy fix if you reread what you wrote. Conjugate the verb by using たがっている. Right now you have first conjugated the tense, then tacked on たがっている, which is why it is funny.
This is a good place to ask casual, or even super technical and high-level questions. We have all sorts of learners on this forum.