My 7 month old baby sleeps on his side the majority of the time. We put him down on his back & he immediately rolls to the side - this occurs regardless if he is drowsy or asleep. He can roll in all directions. Concerned as this seems to be against the always put baby on their back to sleep - but given we do & he immediately rolls thinking this is maybe still ok? Just don't know what we can do/if we need to do anything about it. Thank you

You are correct, the message is to always place baby on back for sleep NOT baby must always sleep on their back.

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 prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.

Once rolling well both ways, ensure baby is moved from bassinet to cot &no; longer wrapped/swaddled.
Baby needs arms free & the space of a cot to roll well.
Ensure mattress in cot is firm & flat.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Once baby is rolling well both ways, place baby on back for sleep but let baby to find own position of comfort, (see article linked above)