I am so sorry to hear this, what a shock for you! Try to stay positive. It's really good that the tumour does not seem to be affecting him and it may respond very positively to treatment.
My friends 5 yr old Welsh had the same, it grew at a rate of knots, and within 3 weeks of it been discovered he's head had swollen. In the end ha had to be PTS. So sad, for a horse so young.
I hope this isn't the case for you, and chemo is an option. She was offered treatment, but at the stage it was at was advised different.