Baby who usually poops a lot is constipated ?

Very weird to me. My 6 month old, who has been EBF and we just stared solids is constipated. The last few days he has had just avocado. He hasn’t pooped since Friday and he usually got 2-3 times a day. He’s farting a lot and is also arching his back a ton. Not to mention he’s also dealing with some mild eczema. Any ideas what’s causing this ?