Jakob has had a skinny upper body for quite a few years. But he balances that by doing weights/cross training -but with an expressed fear of being too muscular (arms and so on).
I don’t know how early Jakob started heat training, but his current videos show a lot of high temperatures training with a lot of clothing. -Want make him less lean I think…
Your question of when he will open his outdoor season: Well, he doesn’t compete in the Nationals the coming week end. That aligns with what he said some time ago -that he probably will need July and half of August to reach a race shape. In one of his last Instagrams he also declared that he is now starting his speed work. So I think your guess about Silesia is a good one…