He is probably lying to you. How could the guitar be lost? You asked him to explain clearly the ins and outs of how he lost it. I used to have classmates who wanted to take something away and deliberately said it was lost, and then they didn't pay all at once and kept putting it off.

Just tell him that your family members ask you about the guitar, and you have to give them an explanation, otherwise they will scold you or something, seriously. Ask him to replace it with a similar guitar, or find your guitar, or pay the money so that you can buy a similar guitar. If he says he has no money or something, ask him to talk to your family, or you can talk to his family.

If you are good friends, he will not do this on purpose. If he really wants to lie to you, there is no need to make friends with such a person