Congratulations to you and your son.
I've only sat on the other side and heard parents speak when their kids graduated. It was always so meaningful to us as parents because as you well know at times you just loose hope that they will ever get through the program. The most touching and helpful were those that described their fears as a parent in the beginning (which is how we all felt) and then praised their child for doing the hard work and fighting the battle (which is something we all dream of being able to say). Its good for the kids to finally hear their parents being proud of them and hopeful for their futures. Some stand up and tell their stories, some write letters to their child, some write poetry, some ad lib - but they all speak from the heart. Why dont you just sit down and randomly write out how you feel right now and then let it take you where your words do.
I'd love to hear what you write when you're done. It is so helpful to see that there is hope.