My baby is almost 8 months old and I’ve been putting him to bed in a sleeping bag (2.5 tog) and also tucking him in firmly with a blanket as the nights get very cool. I make sure the blanket is firmly tucked under the mattress and only goes up to his chest. Is this ok? He seems to like the snug feeling and it makes him sleep on his back rather than rolling around in the cot.

Blankets are fine as long as they are tucked in firmly (not too tight so as to restrain) so there is no risk of his head & face becoming covered.
If he is rolling round & moving out from under the covers, then it is preferable to not use blankets, as they could cover his head &/or face. You could add an extra layer of clothing under his sleeping bag in place of a blanket if he is moving out from under the blanket.
Its not unusual for babies of his age to be rolling all round the cot when sleeping.

Have a look at Red Nose information on bedding for your reference.
https://rednose.org.au/article/how-to-dress-baby-for-sleep