For the shake recipe that Christine has shared, replace the PB with chocolate or caramel spread, both will work really well. If you want to do variations on the chocolate then add some fresh mint or orange zest into the mix.
I started to blend protein bars into my smoothies. Add the liquid first and then the chopped up protein bar (or 2 if you really need the calories), add other things like ice cream, whole milk, whole milk powder or coffee creamer for calories. Also things like chia or flax seeds are high in calories and blend really well. Avocado is fabulous in smoothies too.

I would blend coffee ice cream with milk and caramel Scandishake and a drizzle of caramel syrup for a welcome change on the usual flavours.

It might be worth trying to up the amount of shakes your husband has, if he's only having 3 maybe add a 4th in, if he can manage it.