My baby will be turning 3 months this week, the last 2 weeks he has started turning onto his side in his cot. When I try and put him back on his back he fights back and throws his legs in the air and turns back onto his side, over and over again.. Every night for hours it’s the same. I know it’s not safe for him to sleep on his side, but how do I do this, how do I make him stay on his back?! I feel like I’m going to go insane.

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then will roll & prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Good that he in a cot now, not a bassinet.
Are you still wrapping/swaddling him? You need to cease swaddling/wrapping him now.
He needs the space of a cot & arms free to roll well.

**Ensure the mattress is firm & flat & that there is nothing loose or soft in the cot.
How does he go with tummy time when awake? Is he holding his head up & looking around?
​**Now that he is learning to roll, increase his tummy time when awake to help strengthen his upper body muscles.
Always continue to place baby on back for sleep.

Have a read of the Red Nose information now that he is rolling . 
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

You are not expected to keep waking at night to roll him back, especially as he rolls straight back.
Is he only rolling to his side or also to his tummy? Does he roll back to his back during the night. Is he rolling both ways?

There is no product that is recommended for babies to keep them on their back. Rolling is a normal development stage.
It is all about making their environment safe