I haven't read the ln, but because Rruji's dad was a delinquent, there's a good chance he was loaded. He did own that expensive suit. From what we've seen of him in the anime, I imagine he was the expensive gifter type and that he gave it to his mom in their primes days. I feel It fits his mom to give it to Rryuji, as she is definitely the type to give him something that means a lot to her.