Well that depends if you're following your ped's instructions
If you are, at 9 months, if not, whenever you like
You can start to offer mashed or lumpier food now or in a few weeks. Sooner rather than later probably, or you can get 'stuck' on very smooth purees. As soon as you feel like he might be ready.
You never switch to solids first as such, but you will find he drops a bottle eventually and things then become more fluid, so you might be doing earlymorning bottle, then breakfast solids, then mid morning bottle, then lunch, then afternoon solids, then bedtime bottle.