Any healthy human or dog, who is eating a balanced diet, shouldn't need vitamin supplements.
We humans tend to eat a lot of things that definitely don't make up a 'balanced diet', so some humans might benefit from vitamin supplements.
However we generally ensure that our pets get decent diets, so they're unlikely to benefit from vitamin supplements.
Remember that an excess of some vitamins can be harmful so, if you do decide to give Maxie supplements (which I doubt that he really needs anyway), only give him those specifically intended for dogs (as the human dosage could be harmful to him).