There is no specific way. আপনি just write to him. Start slow though and don't rush him with things about yourself. And be patient, he will be very hesitant, and doesn't usually write back. He will leave আপনি signs he was there to read it though. Make sure to imagine him opening and পাঠ করা the letter, it's a universal thing that will send thought waves to him and tell him to come :)
আপনি could maybe tell him that আপনি believe in him and আপনি can ask him to visit অথবা say you'll always sing and tell him stories, just some suggestions as they ব্যক্ত anything আপনি wanna write really :))
