Dice trays are always nice too, and may have been what he meant when he said ‘mat’. Some are super cheap up to more fancy ones - I’d go for one with his favourite colour! Couple of examples below

Cheap foldable - https://www.amazon.com/Rolling-Storage-Folding-Leather-High-Class/dp/B07MYQZG4M

Fancier wooden- https://www.amazon.com/Forged-Dice-Co-Reversible-Removable/dp/B07JQ9Q896