I'd probably hold off even longer, get as close to 6mo as you can. If he is content after his bottles he's probably fine. Grabbing food and putting it in their mouth I don't really agree with as being a real sign...unless they are doing lots of the other signs because babies mimic you, they see you do it they are interested in you. Plus they are just learning to grab and everything goes in their mouth. But here are some other signs to watch out for
Loss of tongue-thrust reflex - This allows baby to drink and swallow liquids with ease; with the tongue-thrust reflex still present, baby may simply drink in liquid purees or push the food back out. According to Dr. Jim Sears, in the first four months the tongue thrust reflex protects the infant against choking. When any unusual substance is placed on the tongue, it automatically protrudes outward rather than back. Between four and six months this reflex gradually diminishes, giving the glob of cereal a fighting chance of making it from the tongue to the tummy
ready for solids Ability to let you know she is full from a "meal" with signs such as turning away from the bottle or breast. This is important so that baby is able to self-regulate the amount of food being eaten. This helps stop baby from accidentally overeating as parents may continue to feed baby thinking that she is still hungry.
ready for solids Ability to sit up and hold head up unassisted
ready for solids Interest in your food (we tend to disagree with this one as when a baby reaches the age of 4-6 months, he is interested in putting everything in his mouth!)
ready for solids Doubling of birth weight
ready for solids Frequently waking in the middle of the night when a solid sleeping pattern had been established. This may not be the best indicator that your baby is ready for solids. Please keep in mind that a growth spurt will occur between 3-4 months of age, 6-7 months of age and also 9-10 months of age. Baby may also be waking due to an illness or teething.
https://www.wholesomebabyfood.com/readyforsolids.htm