my monster baby has grown (again) but has become slightly too narrow for his saddle does anyone have any ideas as i cant really afford to buy a new saddle, maybe shims? or something similar
Narrow is good - better than saddle being too tight.
I would plug the gap with a prolite / other decent pad for the time being. He is likely to change shape on you again come the spring.
You cannot do anything with a saddle that too narrow for the horse, but the other way around is not that bad with decent padding.
Unless you can buy a wow / changeable gullet saddle (or have oodles of cash), I would be hesitant to go purchasing a new one until they have stopped growing and changing shape.
If your saddle is now too wide then you can use a prolite underneath it to fill the gap - I wouldn't think about getting another saddle yet as he will probably change again so if you can get away with a prolite it will save you money
Use a sheepskin pad or a prolite for the moment. When they have a growth spurt the weight seems to drop away and the saddle ceases to fit. After the spurt has finished he'll start to fill out & the saddle will probably fit him again.