I would say look for a place that matches your values for the kind of environment you want him to be spending time in (how they handle naps, meal times, etc.), how well staff take into account what you're looking for and your/his needs (particularly with things like allergies, etc.), and also just your gut feeling about the place.
We looked at a few and there was really only one that was ever somewhere I would have felt comfortable with. The others were big, anonymous, commercial places. They were much more concerned about money (literally 60% of our meeting with one of them was about all the bad things that would happen to us if we were ever late with a payment!). We visited the baby room in one and the head staff person was like, "before she starts, you'll need to work on breaking her attachment to you so she's easier for us to handle without you here". Uh, WTF? Granted, as she was saying this, she was completely ignoring some poor baby screaming and hyperventilating on the other side of the room, which she explained away as "it's her first day". I literally made it to the car park and burst into tears!! It was awful.
But the place we did choose was nothing like that. They were happy to follow our lead with things we already did at home, like baby-led weaning. They had a very child-led, outdoor learning approach. Unless it's pouring, the kids are outside most of the day. They have a mud kitchen and a forest school. They were happy to do whatever it took to make our daughter's transition a smooth one. Literally for the first 2 months, she wouldn't nap alone there, so someone sat and held her and she slept on their chest for her naps until she was happy to go to sleep on her own. They just really made me feel comfortable that she was being well-looked after and that they were listening to me about what she needed. And a lot of it was just gut instinct. It was a small nursery, lovely staff, based on a farm, so the kids see horses and cows and tractors every day when they're outside. It just felt right. I never had any misgivings about leaving her there and actually have never felt any guilt about being at work because I know she's in a great place and having fun. It's been a wonderful experience. She's been there 2.5 years now.
Also, I wouldn't go just on Ofsted reviews. Read them and understand why they got the review they did and work out if it's a red flag for you or not. The horrible nursery we visited technically has a rating of 'outstanding', but it was a depressing place and I didn't feel like the staff cared very much. The kids didn't even go outside every day. They just had some soft play centre on site that they release them into. I don't think that sounds very 'outstanding' at all.