Issues with 4yr old

Hello IMs
I am having massive issues with my 4.5yr old son. He has always had tantrums, but lately they seem more like complete meltdowns, and totally age inappropriate. Something as little as a cartoon show finishing or no blue straws left for his yakul, or his shoe lace coming undone. I have taken him to a developmental pead, and was told it was behavioural (this was 9mths ago) & go to parenting classes. I have a background in child care, & have done a few positive parenting classes... so not sure where else to turn.
We attend speech therapy once a week. I have also seen an OT regarding his behaviour, and she just referred to another speechie.
He also has serious issues toiletting & in 1.5 yrs since I began toilet training, I can count the number of occasions he has been to the toilet (#2) successfully. I am at my wits end. Today, after encouraging him to sit on the toilet, as I could tell he needed to go, 1min later after telling me it wasnt coming, he stands 1m away from the toilet & poos himself. I've tried charts, rewards, bribery, positive encouragement.. everything I can think of?!
I try so hard to remain patient but after yrs of tantrums and him pooing himself anywhere we go, I am finding myself becoming increasing frustrated, which I know I can direct at him. I feel like I am totally failing as a mum & failing him. But don't know what else to do. He is suppose to start school next yr, but at this stage they won't take him. Don't really know what I am asking, more for advice?