If it's just to move on, there's no need to contact him. You're talking about 35 years, if Jeroen hasn't figured out how to do this, he needs to sit down and put some thought behind it to figure it out.
Yeah, 35+ years make the whole premise suspicious. I hate to say it, but there are enough inconsistencies in the movie that it really doesn't bear much analyzing.
In the first place, I think Walt really loved Jeroen, and it seems impossible to me that he could have simply abandoned him. When he brought Jeroen home that last night, he started to ask the father to tell Jeroen something "tomorrow", but when the language barrier became too much, Walt stopped. He could easily have called Jeroen back out, and made him understand face-to-face what was happening.
When Jeroen got the roll of film developed he found that only one picture came out. The rest had been overexposed when Walt dropped the film and it came undone. Adult Jeroen asked ghost Jeroen on the ferry why he hadn't sent the picture to Hait. The boy answered that while he knew Hait wanted it, he Jeroen wanted to be done with the whole adventure, so he didn't send it. That was unfathomable bitterness for having been abandoned. And adult Jeroen finally understood how deeply he had been wounded.
I think the dog tags made Jeroen do exactly what you suggested. Here at last was a chance to find Walt. But 35+ years (obviously the Best Years of Their Lives) were too much; nothing could rekindle those feelings that blossomed in such distant soil.
But it was great while it lasted...
~~~fia