Tennis is certainly not written off at this age.
He's not, however going to jump straight into a team because that would be miserable for him, and not fair on the opponents.
I played a lot, and still play. I have had success at the local level and play currently for the club teams when I can.
When I was at school, I was often asked why I was so good at tennis (despite being hopeless at other sports).My answer always was that I played a lot. From about age 8 I played 5+ times a week all year round. Yes, I was probably better than average naturally, but the playing a lot took me from being okay to being good.
When playing at school I was often playing against people who played 1-2 times a week through the summer term. Naturally I was going to be better.
But over the years I have seen many people start at various ages-yes even taken up in retirement and achieve a reasonable standard.
If he wants to be good in tennis join a club-both of you. Let him do a coaching session once a week (if you can afford 1-2-1 it will make more difference) to get the technique and invest in a basic bucket filled with (decent) tennis balls. Take him out over the weekend and throw balls at him. 1 bucket forehand, 1 backhand, 1 alternate, 1 volleys, 1 coming into the net (ie 1 baseline, 1 service court, 1 volley), 1 bucket of serves. Do this every weekend when possible throughout the winter. I can't say he will get a team place (or even if the club has teams) next season, but he will be beginning to play a reasonable game.
Tennis is a game that you are not going to start today and (usually) play a full game tomorrow. Many children give up, ironically often the sporty ones, because they expect to walk on, slog the ball and be able to play a game. It feels that you're so far away from a full game at the beginning as there's so much to learn. There still is for me, nearly 40 years after I started, I can still learn new techniques both in court positioning and shots.
The club I played in as a child, my coach used to tell this story. There were a group of teens that came down to play a lot throughout the year, the aim being for fun rather than great tennis. They varied from good downwards and the worst of them was Sam Thompson. Sam started coming down on his own. He'd just come down to the court with a bucket of balls and hit balls across the court, serves, forehands, etc. if if there was no one else there. If anyone came down and was waiting for a partner to turn up, he'd ask them if they would mind having a quick hit with him.
Sam Thompson was men's champion for 20+ years at the club. He didn't have brilliant technique as he'd taught himself, but he had a great game.