@tali26 I think it's perfectly normal to worry - especially considering he was a preemie and started out so vulnerable.
The first few daycare plagues are not pleasant, neither for the children nor for the parents, but it does get easier! I delayed sending my child to nursery to avoid the height of RSV season as we are a lung family, but it's really difficult to avoid all illnesses and they will get sick at some point. All you can do is to try to reduce germs without going to the extent of sterilizing everything in sight.
July/August is thankfully not a bad time to start as there is going to be a bit of time for him to get used to new germs in general before the respiratory virus season starts.
In our family we have certain hygiene measures in place to try to reduce the load of viruses and bacteria that are brought home, but it cannot prevent everything.
In our nursery they request that everyone washes hands when they arrive and leave so of course we do that. However, we are also strict with handwashing when we get home (from anywhere ourside), before meals, diaper changes, wiping a nose, and if hands generally look like they could use a wash. He gets a bath when he gets home and fresh clothes every day. We flush his nose with saline at night.
It's not a perfect system. He still catches stuff and also brings it home on occasion. I had only four days back at work after maternity leave and then spent 2 weeks with flu that turned into pneumonia.
Whichever time you decide is right to take him there, consider having the liquid children's paracetamol ready at home. For some reason the first time they spike a really high fever often seems to be during the night when regular pharmacies are closed. Then you at least have it ready if you need to call a nurse hotline and give baby a bit of relief.