First I'd take a deep breath.
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Secondly, I'd stop offering cake as a treat, at least for now. He doesn't need it, nutritionally speaking, so if he's eating the cake or biscuits, then that's less non-treat nutrition he would be getting.
In some cases food aversions happen. Even at 6+YO, as I've been learning the last couple months (to my own stress LOL). As a child I learned that beans and peas made me literally gag, but that didn't stop my mother from feeding them to me several times a week, including one phase where she was so frustrated that she told me I'd eat the peas or I'd eat nothing. I had nothing but school lunch and glasses of milk for three days before she relented. And I still can't eat peas or lima beans, or most beans except green beans, for that matter, without gagging. I did/do eat a number of other green veg, and beans don't have all that much nutrition you can't find in other places anyway (he can get better protein from meat, for example, or eggs), so looking back there wasn't much point in her having forced the issue except to cause other problems.
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Have you tried letting him be a part of meal preparation? He can probably use a plastic butter knife to cut the sausages into pieces himself, or he can stir eggs or help you make a dressing or dip, or learn to add seasonings to the food. A lot of times kids are more likely to eat something they wouldn't normally if they've had a hand in preparing it.
Does he go food shopping with you? Maybe you can select one new food each week to try. Having food in other settings can also work sometimes; for the longest time Josie wouldn't touch broccoli with a 10-foot pole, BUT she would have it raw with cream cheese at preschool and she'd eat it raw from the garden. She and Nat will also pick beans and peas right off the plants to eat raw, so maybe you could try some of them as raw treats, or serve them with sour cream or ranch dressing as a dip for a new way to eat them.
Also, sometimes he's just not going to be hungry, in which case try again i another 15-20 minutes or so. If you're sure he IS hungry, then give him the stuff you're not sure he'll eat first - so in this case the beans/peas - without the sausage, and THEN let him know there's sausage - otherwise don't even tell him there is any so he doesn't hold out for it. Might be worth a try.
A few random thoughts - hope something in there is useful.