Oh, no proof needed. That's just a possible way to continue this part of the story, and makes sense as long as the developer doesn't decide to keep using the "for educational purposes" excuse to get them (sexually) closer, which is always there and might be the natural path to follow as you say.I'm waiting for proof, but until then I still think that catching Sophia with Ellie(kissing) or doing something for Aiden, would be the best for Dylan, a step forward in Sophia-Dylan relationship.
First of all, L&P can use the same path used before, the love for both children and, this love, has to be the same. Ellie used it for her first french-kiss, touching Sophia's sensitive cord of love for her daughter.
I believe that Dylan will use the same path and, in his case, what he needs is a reason to ask. Right now he doesn't have one, Sophia has kept all her promises (bikini-show, showing breasts, lingerie). There's nothing more to ask.
That's why he's acting like a puppy, doing all kind of "good things" ( e.g. Breakfast in bed ) expecting something from his mother. Catching his mother with Ellie or someone else it would break the barrier between them.
Next step have to be: a kiss/french-kiss or touching her breasts.
Regards