3 year old tantrums....

[Opipjo]

Ds2 has just turned 3, he's generally a happy little chappy. He has delayed speech & understanding.
When we're out and about for eg: Ds1's school had a cake sale on last week. Ds2 just screamed/tantrummed the whole time. He wanted to roam around and do his own thing, i assume. But it was quite busy so he needed to hold my hand.
Walking through town / going to the Doctors / doing the school run even. Something just comes over him and he just strops the whole time.

I'm even getting anxious about taking him places because of it. I praise him to high heaven when he's good and walks/listens etc

DS1 has Autism, myself and SALT don't believe this is the case for DS2..but DS1 also acted like this although we didn't see it so often because those situations rarely arose.

Any tips would be appreciated :)