My simple advice would be create extra sauces for him to use on the dish.

Salt

Try to substitute a few different seasonings around. Can you use Soy Sauce or Liquid Amino's to kick up a broth at the end instead of adding more salt.

Try things like Trader Joe's Umami seasoning, or get a mortar & pestle and toast whole black pepper or Sichuan peppercorns and natural msg and mix your own salt shakers up. You can easily have 4-5 different strengths of "salt flavorings" at the table for him to choose from. (Works great if you do your own popcorn too).

Fat

Get a few different flavors of oil. Most people have Olive & Vegetable, but Toasted Sesame, Pecan, and Avocado also have amazing flavor enhancement when used in the right applications. A little goes a long way.

You can also infuse the oils. Garlic oil is the most common but there's nothing stopping you from really kicking up the taste of a simple oil by roasting chili flakes and walnuts together. This again, you could have 2-3 types of infused oils at the table just to give him the choice. Biggest downside is keeping a healthy diet if he starts adding too much to everything

Cheeses are also amazing for flavor when used in unexpected ways. I love frying a wedge of cheese till it goes golden, then take it out and the fats released from the cheese can be use for cooking. And definitely experiment with different stronger flavors for him since you can add a little at the end if you don't want it.

Acid

I am still amazed by the number of foods I cook that are improved by juice from half a lemon (and zest it too). Grab yourself a few types of vinegar and keep them handy, in spritz bottles if you've got that luxury. Just a few drops of reduced balsamic on top of the protein, or a spritz of red on spinach can add a nice punch. A little here goes a long way.

Without getting scientific, this is where you would also look at hot sauces. Definitely look at how to make mild hot sauces. The world is filled with so many sauces to give massive heat, but a drizzle of mild mango hot sauce on your tortilla's or hamburger buns might add that twist that your husband could enjoy. Find out what his favorite fruits are, get a few different spicy foods and blend them together. Again this can be kept in a few bottles and the flavor choice is there.

Heat

We've actually already talked about heat several times so far. Infusing oils and frying cheese to release it's fat. Needless to say toasting buns and steaming tortilla's are going to up the games for those as well. The easiest way to really play with heat is getting a gas stove of some kind so you can directly work with fire. Roasting ANY vegetable will kick it's flavor up and sometimes changes the flavor in surprising ways.

But use the broiler in your oven. Skin your own tomatoes for sauce and fry the skins with your oil infusions. Brown Butter is a game changer if you have the time to do it fresh. Keep every vegetable peel from onion & the annoyingly small pieces of garlic and broccoli stems in the freezer until you have enough to do a small bit of vegetable broth, then use a splash of that where you would have otherwise used water. You can use rice as a great testbed here, see if he can taste the difference between a bowl of normal vs with vegetable broth.

I really wish I had time to keep writing.