How do you validate someone emotionally when, as you said, he's comparing the intensity of relationships and feels like he's coming up short?
You don't, because
he's the one comparing. So
he has to be the one to validate himself emotionally.
IME, when people learn that skill, they always have their own Validator around because they are it! they can feel solid and secure.
If people look to others to validate them, they always feel at the mercy of others to validate them. If they find one willing to validate? They latch on to their Validator person rather than taking the time to learn the skill themselves. Then they fear the Validator person "disappearing" on them somehow and leaving them stranded. They can't ever relax like that.
You can encourage and you can support. But if he's not willing to stop comparing or change how he compares? You will be talking til you are blue in the face because in the end you cannot do it FOR him.
And he asks, "How do I feel secure when I'm facing that kind of intensity?"
Framed that way? It makes it sound like "secure" is this fleeting fish one must catch from the river. Might catch one, might not. At the mercy of "luck."
Could reframe the question. "When I'm facing that kind of intensity, what do I do/think that TAKES AWAY from my feeling secure? What can I do/think to ADD to my feeling secure?" Framed that way? One CREATES their own security.
I believe feelings ensue after behavior. If he doesn't like how he's feeling, he could examine his behavior.
You can do your part by not gushing at him. Be more even keel like "I enjoy spending time with GF. Thanks for being flexible about sharing my calendar. I appreciate you."
And not gushing like "OMG! I had the BEST time with GF! She is so great and awesome! Wheeeee! I can't wait to schedule her again!"
But in the end? If he's the one comparing, he is the one who stops that comparing. OR he steps it up to compare more realistically and he adjusts for skew. He's not doing realistic comparing right now.
Your relationship with GF is only a few weeks old. He could compare that to when your relationship with him was only a few weeks old. He would probably find that you were pink fluffy lala then too.
Your relationship with BF is now X years old. If/when your GF relationship gets to X years old, it will probably look and feel similar.
Thoughts like that could help ADD to his emotional security.
Galagirl