Sign Language is a living language, it is not something you can "Fail"
If you both like music, try signing to that tune. Thats one of the things we did in our time back in the 80's and still do today. It keeps the language from getting too rusty. Not to mention having to use ASL instead of Signed English to keep up with the likes of Born in the USA etc.
Once you two continue this habit with him filling in on words you dont have until you have the song yourself in sign and enjoy the experience.
Regarding Bullying, send him to Krav Maga. Its a flowing system designed to fight one or 50 people at once. (Not that you are going to last more than 20 seconds against 50. Ive been in real world fights against three and came off with some damage. Knocked two thugs down once and chased a absolute sex offender trying to rape wife in trucking at Jackson TN back in 2000. He got away that time. The offender was invading our cab and had the weight and strength of passion while wife built herself a defensive situation and got ready to fight in the corner of the cab as i came out of the sleeper with a blade. It would have been a mess.
Bullies? Well. Just apply the necessary violence until they stop. Its no fun getting knocked about trying to pick on a pacifist church mouse that seems like they cannot hurt a flower. You would be surprised. We had bullies in my day and we stood up to all of them. SOME quit their bulling ways and frankly actually became valued friends in life. I remember one in particular he used to bully me in high school and years later I ran into him again with him assigned to fix my tractor trailer. It took about a hour of talking before I let him touch the truck. We had a history. When we finished talking we were friends and understood each other. (Hearing versus creepy deaf etc which got cleared up)
As far as depression, you have got to find something he likes to do. Get him into it. Even if it means a day trip. I enjoy bowling for example. I could hardly toss a ball down the lane today what with the medical issues but I get going out there to the alley and throw the damn thing and have FUN. You have to LIVE. Not mope around in a dark room somewhere going aww pity me. Taint so.
IF its women he is depressed about then join the club. This previous year has been very hard on people in terms of being human, social and sometimes engaging in love pursuits and so on. Does alot of damage when you tell people that we all have cooties and you stay way. Yea right.
Employers are crying out for work. Waving wads of money. States are killing off the Federal Money to sit on the couch and so on. Surely he can find something. Now, one does need to be careful, you wont for example catch me working flipping burgers on Kensington Ave in Philladelpia for example. Its a very dangerous and destructive area. I would (And did) move to escape nasty places. Out here in the farmland and forest etc there isnt too many bad people or gangs etc to speak of. There is a whole country out there. Problems begin when say Somalia emigrates to Michigan and refuses to assimilate into Americans. They exclude all but their own. What a waste. You dont want to be working for wages in such a screwed up area.
Ive said enough. But depression? That means you are lonely for something that you enjoy very much. Have him get to it. Get him out of the blues. No medicine needed. Me? Several times Ive been depressed and how I broke that is either be a part of a sailing ship on the Chesapeake for a overnight or take instruction on Steam or Desiel trains in a formal manner as well as learning to operate them on my own money. (Is very expensive but the experience breaks you out of the same old. Thats the key. I finally if nothing else work with horses. I used to do work on breeding farms to stud and take care of the mares and yearlings and so on. The "Children" are a handful and you cannot help but be involved with them and it snaps me out of a dark place.
I can go on and on. But you have to get him moving. Fearing bullying in school or whatever is what stays in school. When you get into the adult world and become for example a licensed carry concealed in public, bulling is the least of any problems. We are way past that. So there is no point being afraid. Its normal to be afraid. That means hes human and everything is fine upstairs.