New to us cat

Mrs_Z

I would be inclined to increase the amount of food.  Just 3,5kg is definitely on the light side for a male cat, although you do say he's small.  He's still young as well, so likely to be active and burn a decent amount of calories. Maybe just introduce another pouch of wet food - so 2 x day, but smaller pouches, 85g each and then either top them with the biscuits or just serve the biscuits for lunch if you're at home during the day.

We have 3 cats and they get 3 meals a day, and there are usually biscuits on the bowls as well.  None of them are fat, but well rounded - or the heavier side of normal I'd say.  One of our boy cats who is rather big (long legs and long body), was about 3.9kg when we got him at the age of 5.  We've now had him almost 5 years and he must be about 4.7kg now - so almost a 1kg heavier.  He was previously an indoor only cat but since coming to us has had access to outdoors and is still very active.  

Rescue cats may well have food issues if they've had to go hungry in their past, so it's best to introduce extra food very carefully.  Also, have a look at the food manufacturer's instructions on the packet - there's usually information about recommended amount according to the cat weight.