I agree with FaeryBee and Therm...the beak is a bit long and could use a trim down by an avian vet if you haven't had any first hand experience doing it. I recommend learning from the vet first to be comfortable in doing it yourself. Although, it appears as a straightforward procedure, there are risks involved.
I would say Simmi's beak is about as long as Zucchini's beak (mah birdie) and he also needs a trim and I am going to have it done by the avian vet, but I also want to learn because I want to be skilled in budgie health care.
I think you are providing Simmi with what he needs but perhaps only one aspect of beak conditioning (beak stone). Cuttlebone isn't a very good beak conditioner. And the beak stone is great for sloughing off beak flakes, sharpening the edges of the beak, but not really for chewing so, you need to get him things to really gnaw on like soft wood pieces, kabob's, cholla perches.
The problem that my budgie Zucchini has is that he was never really a big chewer so I had to give him a variety of wood perches that he could gnaw on at his convenience. Still, he rarely touches those. At the moment, he bruised his beak so he has to wait a couple of weeks before trimming.
Try offering some chewables and give it a week or two. Take new pictures to compare his beak now and in the future and that will give you a better idea if he is utilizing his chew toys effectively. If his beak is still averaging at the same length, then it is a good idea to have it trimmed down by an AV.
Therm gave you a really good picture to see what is good beak length and structure. You can use that, too.