OK what I mean is for example, can you ask him "Do you want a drink?" and he replies "Yes" or "no thank you" or "juice" or "in my pink cup" (etc)
When you say "Look, there's a digger" does he say "Digger!" or "Wow!" (or whatever non-fire-engine thing you might point out).
When you're helping him with things like getting dressed, can you give him instructions like "Arm please" and he will follow them?
Does he make requests from you, like "Again" or "All done" or "That one"?
Does he comment when playing games like if you/he made a tower and it fell down does he say things like "Uh-oh..." "Oh no!" or if you were hiding items "Where is it? There it is!" Does he pretend to count (even if the order of numbers is wrong) or mimic people (e.g. pick up a pretend phone and say "Hello, hello, bye", or pick up a toy frying pan and say things like "Little bit of that..... that's nice..... oooh, lovely")
The above examples are also quite basic for a 3-4 year old, but just as examples of situations where you might expect "typical" speech communication. Because you said that you cannot have a conversation with him, which is not at all typical at preschool age. For reference, all of the examples I listed above are things that my 2.5yo would say, in a situation where he would say them.
In terms of patterns, you mentioned in the OP that when you picked him up from preschool, he is spouting these phrases and when you read him a bedtime story, he only wants Fireman Sam. But I'm thinking about other times of day e.g. mornings before preschool, afternoons after he's been home for a while, weekend days (different times of day). When is he "going into" this pretend game vs when is he using phrases like I asked about above? Or does he ONLY use the Fireman Sam phrases?
You haven't said his age I don't think - I am guessing if he is at preschool, he's 3 or 4?