Your baby is delay in growing, yes is little delayed he must be at least above 80 cm. It can be many reason like he may suffering from any kind of health condition such as growth hormone deficiency, or hypothyroidism, heredity, etc. To find exact reason better you once do all physical examination on him and do all necessary test like thyroid function, vitamins level (all vitamins. Calcium, mineral check.
From 6 months onwards he can be given homemade mashed foods and should never be given in bottle with nipple and should be fed with a spoon. In fcat ,f the milk is less, the semi solids can be started from 4th months onwards.
Your baby needs a thorough workup and need to check for hypothyroidism and if it its there she needs thyroid supplements if bottle fed, you need to immediately stop bottle and feed from a cup or glass with spoon.