Adjusting the fit for a rounder tummy

My boyfriend measures at the lower end of XL for neck and chest but just over the XXL waist size. How relaxed is the fit round the tummy? I feel like i need to grade to the XXL around the tummy then add a smidge more as i don't want him to be self conscious. What's the best way to do this? I am doing a toile so i can have a play.