It is recommended to give semi solids after 6 months
If you are just breastfeeding then there is nothing to worry. Also if baby is thin but his weight and height is OK then there is nothing is worry.
You can also talk to your doctor and he or she can suggest you formula milk if needed.
After 6 months start with daal ka paani, daliya and curd etc
Always consult with your doctor before starting anything.