OK, first of all, I'm not judging you. My 9 y/o DD has always been slightly on the high end of normal, & was overweight last year but we've got her back into a more healthy weight for her height now. And she is awaiting ASD assessment as well. So I understand some of the challenges you may feel.
How wedded is he to the brioche for breakfast? It's really not great. Would he go for scrambled egg on toast? Or porridge with a small spoonful of honey or jam on, or a sliced banana or some berries? Both are much more nutritious choices, have more fibre, protein & will keep him full for longer. At the very least, if he won't give up the brioche, can you compromise on 1 piece only, with fruit & a glass of milk?
You need to cut right down on the snacks & in between meals stuff. So does his father. Humans aren't cows, we don't need to graze. Occasionally feeling a bit of hunger won't harm him, & it will probably encourage him to finish his meals when he's given them, hopefully including the vegetables. How is he with vegetables?
Get him into as much exercise as possible. Conscious & subconscious. Go to the park or on a walk, but also just walk sometimes instead of taking the car / public transport. Get him doing jobs for you to earn pocket money - vacuuming, wash the car, mop the kitchen floor, anything safe but a bit strenuous.
I wouldn't necessarily switch to skimmed milk. Cutting down/out the junk food & sweets, & increasing his exercise is much more likely to be beneficial to his health than reducing fat intake. Fat is needed for healthy growth & to satisfy his appetite. You need to massively reduce the refined carbs - all the pastries etc need to be cut out, they should be occasional treats, not daily staples. He's filling up on them but because they aren't filling he's hungry again really quickly. More protein, vegetables, unrefined carbs to replace the refined stuff - potato wedges, rice, even regular pasta.
Regarding reducing the sweets etc in your DS's diet, we did talk to our DD about it, but we framed it as being about all trying to get more healthy together, & eating less sugar because it's bad for our teeth. This is something they'd previously covered in school a few years ago, so it wasn't new to her.
Don't be ashamed. You're sorting it. 