Fermented vegetables can often provide the pungent, salty kick that is often desired from mustard. If he is not allergic to cabbage (which is a member of the same family, brassica), sauerkraut may be a good option. Similarly, other members of the brassica family, such as broccoli and collard greens, retain a similar bite, particularly when raw. You might try making a collard green pesto.
Outside the brassica family, tamarinds can deliver a pungent, sour flavor that complements many different kinds of foods. Tamarind is a signature ingredient in many sauces and chutneys, particularly for meat. You could try making your own A1 sauce while leaving out the mustard.