DS is currently 15 weeks old and weighs near 7kg.
He is on formula and has been since three weeks old.
He has 180ml bottles every 3 and a half-4 hours.
My question is; could the formula not be filling him up enough and that's why he drinks so much? we are currently giving him SMA formula but am willing to try anything to help my big boy cut down a bit.

This may be normal i am just a tad worried as DD (19 months) never drank this much when she was on formula and now only weighs a shy of 11.5kg

Any advice will be much appreciated

If you are using and mixing the formula correctly, then your baby is fine. Kids grow at different rates - even siblings. He will slow down eventually.

The only reason kids might get 'overweght' on formula is when they are force fed it e.g. with a syringe, or mums want to give their kids 'extra' nutrition and use too many scoops of powder per volume of water.

Don't try and cut it down, kids under 1 are remarkable at regulating their intake to their needs. My DS (now 2.5 and 13 kg was drinking about 1100 mL in total of formula a day at that age - which is about 180 ml every 4 hours. At almost 4 months, he is also likely to be going through a growth spurt - babies get fat then suddenly long. This is normal.

The only other question I would ask is whether he is getting enough sleep. At four months, he should still be sleeping about 15-16 hours a day (on average, some babies do more, some less) in total including night-time and naps. If he is not getting enough sleep, he will be compensating for the lack of energy you get by sleeping by trying to get energy from food.

He could well just be a big bub - boys do tend to be bigger than girls if my 4 month old DS (BF) is anything to go by, pushing 8kg here and long as well. DD was quite petite at the same age by comparison.

My mothers group has a mix of FF and BF bubs and they're all around the 6-8kg mark. I'd keep on as you are but maybe ask your CHN or GP to double check.

Thank you all

He has about 3-4 small naps during the day (1hr) and most nights will sleep about 6 hours, other than that he is awake and trying to explore the world.

Once again thank you so much.

Before changing his diet talk to a health professional. He might be well within the normal range and you shouldn't put a baby on a diet!
Once he starts crawling and moving he'll slim down anyway, mine did(though BF).