Hi, My little boy is 13 months old and just started rolling. Not confidently yet but he can crawl. He always seem to sleep better on his tummy, is it safe at this time for me to put him to sleep on his tummy?

At 13mths of age, your baby is out of the age range for SUDI (Sudden Unexpected Death in Infancy - babies less than 12 mths)
The general recommendations for babies once they are rolling are:
Still always place baby on back for sleep, & when rolling well, can be left in tummy or side position.
Cease swaddling & move to cot(if still in bassinet) Babies needs arms free & space of a cot to roll safely.

Red Nose does not recommend settling babies on their tummy, especially if they are not rolling well both way
Here is general information for babies who are rolling:
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over