cxar la leciono estas tre longega!
Why not simply 'finos'? Sounds like translated English to me.
Then a correct translation would be "He finishes … tomorrow" This is where Esperanto can get very subtle.
Well, more accurately "finos" is "will finish" while "estos finanta" is "will be finishing". "Finishing" and "finanta" here are active participles and they exist so the course is teaching them to you.
I thought it was: He will have finished the lesson tomorrow.
That would be "estos fininta" instead of "estos finanta"
kaj mi finisxos la kurso en ses tagoj!
How about "Li finantos la lecionon morgaŭ." ?