I'm not going to say it's impossible, but it's very unlikely. What you're looking at is taming a reptile in a way you would if you received one that's been mistreated or one that wasn't captive-bred. Pretty much all methods rely on you having the reptile in your immediate care, and having it rely on you entirely.
First and foremost, taming works best when the animal is a baby. Since he's already full grown, it's going to take a bit more patience.
On the other hand, Blue-Tongued Skinks are regarded as being one of the most friendly lizards. I have seen arguments occur between which is friendlier, a Blue-Tongued Skink or a Bearded Dragon. So there is something in your favor.
It seems as though he's starting to be comfortable around you. But it sounds like he still sees you as a possible threat. This might not ever change since he's still a wild animal. The best way you can solve this is spending as much time with him as possible. This is where if he was in an enclosure, you would throw a worn t-shirt, or something into it to get him used to your scent.
If you're looking to tame him, you need to offer him food. Sort of like a stray cat, start by just having him accept food that you offer him. Place some food down, give him some space. If he eats around you that shows he's comfortable. Slowly (as in each day) start inching closer each time you feed him. Eventually, you should be able to offer him food right next to you. At this point off food from your hand.
Now, with that being said, you should take into account the side effects of feeding wild animals. As you work to tame him, he'll start to rely on you for food. he will stop looking for food on his own and start to expect food from you. This might not be a problem at the moment, but how will he get food if you go on vacation, or move away? I always say not to feed wild animals unless you plan on taking care of the completely - For the sake of their independance.
Unless you're planning on taking him in as a pet. The best thing you can do is just be present. He will start to understand that you aren't a threat to him.