My 2 year old is growing out of his cot so we are looking at putting him into a bed. Our concern is that while he sleeps on his back and side, most of the time he sleeps on his stomach, even in the warmer weather. I’m unsure what to do in regards to a pillow and/or training him to sleep on his side/back. Any advice would be appreciated

Red Nose Safe Sleeping guidelines are specifically aimed at reducing the risk of SUDI (Sudden Unexpected Death in Infancy - SIDS & fatal sleep accidents) which occurs in babies less than 12 months of age.
However, Red Nose also provides information/education to reduce the risk of sleeping accidents in older age groups.

Most babies will be rolling by 4-5 months of age & many prefer to sleep on their side or tummy.
This is a very normal stage of development.
Always place baby on their back for sleep, but once they are rolling safely, it is not necessary to roll them back or “train him to sleep on his side/back”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Red Nose does recommend withholding the use of a pillow until child is no longer in a cot.  However, many children sleep quite well without a pillow.

When transferring a child from a cot to a bed, there are a number of safety issues to consider.
Have a browse through the information linked below about choice of bed type & home safety.
https://rednose.org.au/article/where-to-sleep-a-child-who-has-moved-from-a-cot
https://rednose.org.au/article/making-your-home-safe-before-moving-a-child-from-a-cot-to-a-bed
I have also attached Cot to Bed brochure.