Hey Claire, replied on the July thread before I saw your post here, so lets see - here goes. Sorry if it's long!
*I'm going to assume a lot now

*
I'm assuming that you didn't introduce solids (as in pieces of food) in this way and that you only give it to Colby every once in a while but still feed purity when you feel he's not eating enough.
The thing with spoon feeding is usually that they haven't really learned to chew because they still just suck down the purity like a liquid, or they might 'chew' chunks in the purity (if you are on stage 3) but it's a lot less work than biting off pieces of food and chewing it.
The only advice I can really give is to start slowly by replacing a meal (assuming that you already give finger food snacks) and not top it up with purity or baby cereals. Let him get use to 'working' for his lunch.
More advice would be to give 3 different things, one of each and that's it until he finished it then he can have more. (Example) one finger shaped piece of cheese, 4th of a plum and stick of butternut.
What I normally did is to eat with B to show her how to chew, she enjoys copying us, if she eats only her butternut stick she gets another. If she doesn't eat the plum then so be it - no pressure. But one thing always remained the same, if she didn't eat then she didn't get anything else, I never topped her up on purees because I felt that she didn't eat well enough. Usually when I thought that she ate so little that we would have to get up every 2 hours at night to feed her, she'd sleep through.
On a daily basis B has:
For breakfast: 1 block weetbix (the kind with no added sugar or salt) with full cream regular milk. I make it just wet enough not to be sticky, preload the spoon and she guides the spoon to her mouth.
For a snack she has Bites (that's the name of the stuff) flings she has 6 or 7 max, otees or 1 tennis biscuit because she doesn't like the marie biscuits with her top teeth coming through, she loves cheese and with this she has rooibos tea with milk (or should I say milk with tea)
For Lunch any of the following, just one sort or if I feel like it 2 or 3 different things: Scrambled eggs, plain pasta, cucumber slices, cheese sticks, yoghurt, toast
For dinner really anything we eat, which usually is fish/mince/chops/chicken, potatoes/rice and veggies from peas to butternut. She gets about 3 pieces of each different type of food.
Her favourite things at the moment are cheese, cucumber, rice, peas and chicken
Oh yes, Edited to add - things to avoid would be nuts and any 'hard' type of food that doesn't get soft by adding saliva an example of that would be simba/lays 'hard' type chips because it only gets soft after really chewing it - I once had a moment with a simba chip and would not recommend it. And keep the sugar/salt and msg's to a minimum. Also remove small pips/stones/seeds from fruit.