Send them tickets or a gift card to do something:
Movies
Museum
Pottery Place
Kid-themed restaurant (Dave & Busters)
If you knew their mother or could share a story with them, include that in the card. Otherwise, just send them a "thinking of you" card along with the gift certificates.
You can also call a local restaurant and explain the situation. Ask what they would charge to have dinner delivered to the house or call a diner and ask if they could do a weeks worth of "brown bag lunches" from you. You know, sandwiches, chips, dessert, fruit cup, etc that dad could redeem when needed- one less thing for him to worry about.
When my nephew died at age 2 my BIL's company did this and it was a really wonderful idea. All they had to do was call the local diner and say "we need two lunches for tomorrow" and they picked them up (prepackaged and ready to go) 15 minutes later.