How low is my cats immune system after sickness?

Background: my 9 month old kitten who was vaccinated got panleuk when he was 6mo back in December. It could’ve been from the sick kittens I fostered or from him being exposed to our shoes during trails and hikes.

He has since recovered and his blood work is back to being amazing and they told me he would have a low immune system. They said I could still foster but I’m extremely hesitant of getting him sick again.

Before getting sick, he would go for walks with me around the block on a leash to check the mail- 15 minute walk.

My questions are:

1. How can I tell how low his immune system is? His blood work returned to normal.
2. Is he still safe to go for walks on leashes or does the ground pose a possible threat for his lowered immune system?

I’m in the US, husband and I live a few minutes away from our VET. No worry of cost.