Ok, reading your post this is my perception of your ds, which may be wrong.
Your ds is good at drawing TRAINS. My ds1 (age just 7, Y2) is good at drawing pirate ships and Captain Jack Sparrow and Buzz Lightyear. My ds1 is not 'gifted' at drawing, but he does love pirates of the caribbean and toy story and as a result has practised drawing them - he draws them all the time. This is great - he has written POTC books, illustrated them, which helps his writing skills, his fine motor skills, his perception of stories etc. If you asked him to draw anything else he would have a go, and it would be ok, but not as outstanding as his pirate ships and Buzz Lightyears.
However if at school he only drew these things, he might be laughed at - oh look, it's ANOTHER pirate ship. (In fact, he does draw them lots at school, but they are ALL into pirates...). A friend of his DID have the experience of being laughed at for ALWAYS drawing trains, and tried space rockets instead. And if a child is laughed at they might try something else.
So is it possible that your child ALWAYS drew trains, which he has practised lots and as a result are beyond the rest of his drawing skills, the children at school noticed and laughed, so he tried something else, which he can't draw yet, and it looks like scribble? Or the teacher asked them to draw 'a seaside' or 'a park' and because it isn't a train, it looks like scribble? In which case, the worst thing you can do is to say 'this is scribble' (I know you didn't, but ykwim). And also, it is not dumbing down - scribbling is a very important part of development - the feeling of the pencil across the paper, the movement and the resulting marks made.
Children can very often draw ONE thing very well - I don't think this means they are G&T, just that they like doing it and have done it lots!!! I really wouldn't speak to the teacher, because you don't know what they set out to do in the drawings you don't rate. And he does have to learn to draw things other than trains!
Ds1 interestingly is very good at the piano too!