Conure and birdy num-nums

Sherlock is a recently adopted five and a half year old Cinnamon Green Cheek Conure who refuses to eat anything remotely healthy - he only eats sunflower seeds, hard corn kernels and few other seeds and REFUSES to eat any fresh fruit or vegetables, any dietary pellets or parrot mixes, and is extremely fat as a result.
In fact he’s the fattest little Conure I’ve ever seen.
I adore this little guy - he’s only been with me a month and a friend helped me adopt him to cope with the unexpected passing of my previous Green Cheek, Napoleon Solo, who passed away during a routine medical procedure, leaving me devastated.
At first Sherlock was a bitey, horrible little monster (which is pretty standard for any adult parrot in a new, unfamiliar environment) but after a surprisingly short period of time he has bonded with me and we get along like a veritable house on fire.
But his unhealthy preference for sunflower seeds is driving me nuts and I’m trying to limit his intake of them - although when I do he ends up not eating bloody anything.
He will often eat stuff he sees me eating - except he seems to be able to tell when I’m trying to get him to eat healthy parrot food by pretending to enjoy it myself.
The only fruit I’ve managed to get him to eat is organic baby food and the occasional berry smoothies.
This evening however he doesn’t seem interested in the five berry smoothie I made especially for him.