my son did the same thing. He ate all the meals throughout the day so well, but by the time we got to dinner he was very very picky and ate little. Yes, it is a phase and I wouldn't worry too much about it. Kids don't let themselves starve. So if you're seeing regular diapers and weight gain, I wouldn't really do much about it. There were two things though that I noticed when we went through this and they were:
1. if I fed him the same veggie every night for a week, he refused it the next week. He simply wants variety like we do.
2. I was attempting to have him eat dinner with us, so that dinner was a family event, but 6:30 (which is what time it was by the time we were all home and dinner was ready) was just too late for him. So now I feed him between 5 - 5:30 and he's been finishing his plate.
Maybe consider those things, but in general I don't think it's something to be alarmed about, especially if he's still on formula or breast milk.
K.