I start solid at 5 month old when my baby able to sit up. the min requirements is baby able to sit up. https://www.instagram.com/p/CAcrS_gja9F/?igshid=i4kii4fubxlo here's a little guide hope it helpa
It might be his growth spurt. Talk to your pd to see if your baby can start solid early. For my case as my baby is not eating well and her weight is a concern, the pd ask us to start solid early.