Other national standard variants are certainly a good start. Do not neglect
the 8x8 ones, as they are easiest to play with standard FIDE equipment.
Beyond that I would reconmmend the variants that have most influenced me -
Bachelor, Fusion, Wildebeest, and Yang Qi come to mind immediately. If he
gets hooked he might like to try variants which have sets available for
sale, such as Thud.