That could be the issue, you have taught him to go wee, but not poo if this makes sense.
All mine go on command, they hold it until i say so, but take them out at regular intervals.
If you are around when he poo's teach him the poo command, then he will get the gist of things with both.
Maybe use a gerneral term for both, I let each one out and say "get busy". Before bed i Say come one get busy, they do a wee, then i say get busy and they will do a poo.
Hope this makes sense, I think you have given good boundries with the wee command, but he is all over the place with his poo's because you havent given any guidelines chick.
Hope my waffle makes sense hun
xx

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Sarah xxx