For a baby that small I'd say a short sleeved vest and a 1 tog sleeping bag. My girl is 11 months old and her room is just under 24 and this is what she's wearing. I only had her in a vest when our room reached 27 last summer!
Please don’t put a fan on near the crib/ cot. Fans should always be facing away from baby and usually the opposite side of a room. My girl is in a long sleeve vest with a 1 tog sleeping bag and her room is currently 22.9 (was 24 going to bed). I don’t think a vest on its own would suffice for a 7 week old as they can’t regulate their body heat. We took our little one to Tenerife at 8 weeks old and when the room was 24 we just used a nappy and 1 tog swaddle bag (probably equivalent to a sleepsuit)