I will definitely pray for him and his parents.
I can personally relate to his struggles because I spent years questioning my salvation. I still to this day to not know the exact moment I was saved but I know that I am. I've talked to others that struggle with not knowing the exact moment as many seem to. God works with people differently so some people can say at 4:58 on Tuesday the 24th I was saved. Others can't.
I like to think of my salvation like hand forging a knife. It was a slow, methodical process that resulted in a blade. It didn't start out sharp(why I don't know the exact moment) but over the course of the process it got sharper and finer. The knife was clearly visible through the process but the sharpness was not there at the start so many, including myself, may not call it a blade or a knife.
Again, from experience....if he is questioning his salvation that shows that it's important to him. He has conviction, either to you and what you've taught him or to God. Neither one in my eyes is bad because it shows he values your instruction and at the very minimum he values God. Questioning God or question salvation is not unbiblical....Paul questioned himself in Romans 7.
14For we know that the Law is spiritual, but I am of flesh, sold into bondage to sin.
15For what I am doing, I do not understand; for I am not practicing what I
would like to
do, but I am doing the very thing I hate.
16But if I do the very thing I do not want
to do, I agree with the Law,
confessing that the Law is good.
17So now, no longer am I the one doing it, but sin which dwells in me.
18For I know that nothing good dwells in me, that is, in my flesh; for the willing is present in me, but the doing of the good
is not.
19For the good that I want, I do not do, but I practice the very evil that I do not want.
20But if I am doing the very thing I do not want, I am no longer the one doing it, but sin which dwells in me.
21I find then the principle that evil is present in me, the one who wants to do good.
22For I joyfully concur with the law of God in the inner man,
23but I see a different law in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin which is in my members.
24Wretched man that I am! Who will set me free from the body of this death?
25Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin.