feeding troubles

Slmz. I started my 6 month old baby on solids. He is enjoying his cereal and now for lunch I made butternut for him. He does not really like it. He ended up crying with every bite he took. I'm so confused as to what to give him now. He is slightly under weight as well. so I'm very worried. He does not look under weight and is not sickly or anything like that. And also how much of the veggies should I be giving him? I don't want to underfeed him. Any help would be very much appreciated.jia.