Sir, my baby is 4 and half months and his weight is 6.5kg , can i start solid food now?

Can or should: You can & will do whatever you want. Breastmilk or formula supplies all the nutrients an infant needs. Most suggest they start when the kid can sit & participate in the feed instead of having it pushed on them. That is a point where the tongue thrust is fading & the transition to all table foods can begin in earnest. Solids are less balanced nutritionally in most cases than BM/F.