Oh lollycat. Where to start abuot boy cats and outside access?
I'm afraid mine would never be allowed out until they were 21 and in the company of a responsible adult, but that's just me!
All of our cats have indoor and outdoor access, but having lost 4 cats to the roads in a short space of time, and knowing of two neighbours cats who were killed and injured, it's a fraught topic for me. Much depends how near you are to potential hazards, like busy main roads, rat runs, or the like.
I would suggest first of all only letting him out when you can supervise him. Secondly, let him out when he's hungry and not been fed, so you can coax him in using food treats (assuming he's food motivated).
If you can, get him to wear a good, well fitting safety collar - the breakaway variety, with a disc and a contact telephone number. If he gets out of them easily, you may need to buy in a supply.
Try playing with him in the garden, using an interactive toy, such as a wand toy, encouraging him to hunt and play with the toy. This will need to go on for some time, so you can tire him out, but he will like as not be distracted by sounds, scents and sights to investigate, so you need to persevere.
If you can make your garden as interesting as possible, it will help. We made a wood pile for ours, as they like to watch the mice. They also have a pond, where they can sit and watch the frogs.
You may find you're having to encroach on neighbour's property and in their gardens trying to get your boy to come back if you can't engage him in play, or get him to come back with treats. It can be heart-stopping and frustrating.
Kittens have no sense, especially so on roads. The Bristol Cat Study has shown most cats who are killed on the roads are killed within the first two years of their lives. Harsh, but true.
Your little boy sounds like he's adventurous, and inclined to explore. I would try and keep him in when you're not around to supervise him, and also when it's getting dark in the months approaching winter. It won't be easy as cats love to be out in the dark.
You can get catflaps which will read microchips and allow some cats to be in/out and others to be on curfew. We have a Sureflap dualscan flap which has this facility. They aren't cheap, but they're worth their weight in gold when you're kitten training.
Good luck, and I hope that both your little ones will be safe whilst exploring, and learning about their environment.