There are a lot of ways you could go about this. One is of course writing the letter as a eulogy but as spoken to Guido, the father. Or, just write the letter in typical epistolary form. It might be interesting to include what you (Joshua) are up to in terms of career, relationships, where you live and so on. Or maybe you have children of your own and you can detail a story where you used imagination and storytelling to make a difficult situation bearable. Given that you are writing this letter to thank him for what he did, you are essentially thanking him for love and creativity. So, I would recommend being creative in your letter; and this can be in both content and style. Mix it up; throw a poem in if you want. It would be fitting if your letter was in some kind of story form since that was the gift Guido gave Joshua.
One idea: Recount the victory you had with Guido, winning the tank and then go on to tell an embellished, fantastical story (like Guido did) of some triumph you’ve had since Guido’s death, following that with what actually happened.