My baby is 7 months and started to sleep on his stomach every time I put him on sleep. I'm concerned and please advice me on what to do.

What you are describing is normal development.
Most babies will be rolling over by 4 - 6 months…. some earlier, some later
This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy, & often don’t change from that position.

This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur.

Always place baby on their back to sleep.

Transfer to cot if not already.

Ensure cot mattress is firm, well-fitting & flat.

Ensure there is nothing loose or soft in the cot.

Use a sleeping bag now that has appropriate size neck & arm holes if appropriate….. cease wrapping if you have not already done so.

It is essential to discontinue wrapping as soon as baby starts showing signs that they can begin to roll..
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Red Nose has the following information now that your baby is starting to roll.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lamb’s wool, doonas, pillows, cot bumpers or soft toys)
As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position

https://rednose.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

The information about baby rolling is the same, regardless of age.
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 their new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Is you baby still in bassinet or a cot now?
Is your baby still being wrapped/swaddled.
Babies who are rolling, need to space of a cot & arms free to roll well.

How does your baby go with tummy time when awake?
Always, continue to place baby on their back for sleep.

Most babies will be rolling by 4 - 5 mths.
This is a normal development stage.

Once baby is rolling (or attempting to roll) swaddling should be ceased & baby moved to cot.
Babies need arms free & space of cot to roll well & unhindered.
If baby is unable to roll both wells freely, then the should gently be rolled to their back.
Increase tummy time when baby is awake to help build upper body strength which will assist with rolling.

As your baby is rolling confidently, still place her on her back for sleep & let her find her own position of comfort as long as she is in a cot(not bassinet) & is no longer swaddled (must have arms free)

Hi Pru

Most babies will be rolling by 4 - 5mths of age.
This is a normal development period.

Is he in a cot now (not bassinet) & no longer swaddled (arms free)?
Is the cot mattress firm & flat?
Ensure nothing is loose or soft in cot.
Does he hold his head well when on his tummy & reach for toys?
Still always place him on his back for sleep.

Are you using blankets or only a sleeping bag?
Ensure his head & face remain uncovered.
If baby can roll well, both ways, place him on back for sleep & allow him to find his position of comfort.