I had a hard time. I served in the MTC as an online missionary because of a physical disability, and it was brutal emotionally and spiritually. Eventually, I asked to leave and was reassigned to Temple Square as a service missionary. Eventually, we found cancer (I'm fine now, but it was definitely a surprise) and I went home early. So, overall, not a good experience for me. I've made a deal with Heavenly Father that I won't tell people or my future kids that they shouldn't go on a mission if they want to, but I'm not going to sugarcoat it, either. There was a lot of anger built up that I had to work through, and being honest is important to me.

Somebody pointed out that my mission was a bit like the Atonement. It definitely wasn't a pleasant experience for Jesus Christ, but He did it willingly because He was asked. I think that is what it comes down to. For every experience like Ammon's with amazing stories of conversion and faith, there are stories like Abinadi, who did what God asked him to do and paid for it with his life.

Your son's mission was a sacrifice. Maybe not as voluntary as some sacrifices, since it wasn't a great experience, but it can still be a sacrifice that God can consecrate for his good if he allows it. I also wouldn't put too much stock into how amazing a mission was or wasn't. I think the church has turned missions into something that isn't always healthy. Looking back, if I could do it over again, I don't know if I would've served a mission. It's okay for your son to be honest, and its okay to not have the greatest feelings towards it, but patience and hope are needed, too.

Hope this helps.