You have many responses, and they may be helpful if your child turns out to be truly having a problem. However, what you seem to be saying is you went from exclusive breastfeeding to three meals a day of solids at only 6 months old.
There is no magic number of 6 months that says this is the right time for a baby's digestive system to take solids, especially so much all at once. In fact, many nutritional and developmental experts would say that 6 months is the minimum age that even beginning solids should happen. This needs to be very gradually done, with maybe an entire meal--often some type of rice cereal mixed with breastmilk or a non-constipating juice, or mashed banana or avocado being the best initial choices--being offered ONE time a day, certainly not three yet.
I know it is exciting to offer solid foods--it's amazing to have your baby discovering new tastes and foods! And it can feel like it's important to have him on solids quickly.
But really, his little digestive system seems to be signalling pretty strongly to you that it is not yet ready for this level of things.
Your best bet would be to back off completely from solids for a few days--if he wasn't constipated before with exclusive breastfeeding, it's likely this alone will solve the problem. Then, very gradually, add back ONE solid food at a time. I really like bananas or avocados for this first, because they are so complete nutritionally, because they come in a single, natural "package" that has been used for literally hundreds, if not thousands, of years as baby's first foods, rather than being a blend formulated by adults with some concept of what might work. They also contain natural oils and digestive enzymes that will help his digestive system handle them. PLEASE splurge for the organic versions, if they are available where you live--non-organic bananas and avocados are treated with a systemic poison that is in the fruit, not just on the skin.
Each new food from there should be added one at a time, with at least a few days to a week to see how he handles it before you add another. If at any point he seems to have a reaction such as constipation or rash or any excessive fussiness/crying, remove the latest food and see if it gets better. You basically get to play "scientist" and conduct a very strategic process as you and your baby learn this exciting new world of digestive processes together.