I mentioned the other day that I'm not that convinced that he has tried to walk to his girlfriend's a couple reasons why.
First the clothes he was reported to be wearing when he left, would be nowhere near warm enough for spending time outside at night, also he would need supplies food, water, ect.
Now I suppose he could have hid a rucksack with all the suff he would need if he had planned this, but I have not seen it reported that anything else's was missing form his house eg, extra clothes or food.
Also it has been reported that his phone had hardly any charge on it when he left the house so I'm not sure how he would manage to be able to get all that way without Google maps, I suppose he could have a old school map but how many 16 year olds would have one?
He would also have to take the A and B roads so he would definitely need some form of getting directions without getting completely lost. I have driven all over the country with work and I don't think I would be able to do that journey from memory, especially since he wouldn't be able to walk along the motorway for very long without getting picked up by LE.
However I suppose he might have tired to walk that far, as you said who knows what a 16 year old would try, it's been a long time since I was 16.