Have you unlocked the Henshinjo destination yet? You need to have gotten all four TamaTomo seals for 4 unique (for example, if you've gotten TamaTomos for Mametchi, Kuchipatchi, Kuchipatchi, Furawatchi, the destination will not open up until you get one more different character) characters before that destination opens up. After that happens, you need to have purchased the crown (the item necessary for Mametchi's alter ego transformation) from Tama Mori and have it in your item chest when you take Mametchi to the Henshinjo in order for him to transform.
Hm I have the crown and he just did a little dance when he got all four happy symbols, I think my problem is he is only my first adult? I need to get 3 more special characters with all there happy symbols?
If this is true I should just marry him off now anyway yeah?
Ah, he's your first adult character, I see. Yes, you need 3 more characters (doesn't need to be the special characters that transform) with all their happy symbols to unlock the Henshinjo. Marrying him off would be the option to take. The more characters you get, the more destinations (last destination requires obtaining all 32 characters, just to be clear...there was some confusion in that post) you can unlock.