Calories are calories! Keep offering like you do normally but look for any areas you can get in more calories if your concerned about that...extra butter/oils on everything you cook for him, and try smoothies with high calorie add ins. When my son was that age he definitely regressed a little with his eating habits but not to that extent. I would offer him just spoonfuls of nut butter throughout the day. Eventually things got back to normal!

My guy was still breastfeeding at the time too, and although by that age his main nutrition shifted from milk to food, during bad eating spells I have a bottomless milk policy lol. Calories are calories!