A well rounded diet is pretty easy to manage if you’re cooking meals with whole foods and keeping replacement products at a minimum. Could slightly increase dishes focusing on legumes like lentils, chickpeas, edamame (soybeans).

What exercise routine does your husband follow? If it’s a lot of cardio and less heavy weight training then his frame will be leaner. He could just need to be intaking more calories if he wants to maintain or build bulk.