You are disobeying instructions from your manager. Stop doing that.
my manager asks me for a feedback on his behavior but ...
There is no but. From your comment it is clear that there may be a backstory you are not aware of. It's quite possible this behavior is something that has been formally raised (e.g. via HR) with this employee before and their behavior is now under close scrutiny and a performance goal must be met by that employee.
Whatever is going on (and you may not know the full story), your sole responsibility is to manage the empolyee as normal, as you would any other staff member and report the information your boss requested to him as a neutral observer.
So your job.
... I keep delaying it because if I tell him of these flaws, he will fire him immediately.
This is outside of your area of responsibility and you probably do not fully understand the reasons. But note that your failure to do your own job as ordered is putting your own position at risk.
Do not second guess why, just do as asked unless, of course, you know it's an illegal order or forbidden by HR policies. In either of those cases you ask for written clarification/confirmation of your orders (to cover yourself).
Now I know he belongs to a very poor family and I don't want to be the reason of his dismissal.
And I bet you know people who belong to poor families and manage to do their work and behave reasonably. Their reasons are not your concern. If anyone is required to worry about the employee's social or personal issues it will be HR. Again let them do their job and do yours.
Having to be the one involved in the process that may result in someone being fired is certainly uneasy for most people - it's actually good this bothers you, in some ways. But someone has to take responsibility for these matters and your small role in this (potential !) termination is your duty by your employers.
I have already tried to tell him of these issues and that I am unhappy with it but doesn't seem to care and still does them anyway.
Report that. Let your manager make their own judgement call with whatever information they have.
When I look at his technical skills, he's above average and gets his job done timely.
You are free to express that in your communication to your manager, but you also need to report the negative stuff. Do not let yourself bias your report. Straight facts, no embellishment, no distortions. That's the way you fairly serve both parties.