Boy's Christening outfit - top half and bottom half different sizes

[GatsbysGirl]

DS2 has his Christening booked in November. At 9 months old he is currently wears 12-18 month tops and 9-12 month bottoms.
The church we attend is not known for it's warmth so I don't want him in short sleeves while most of the congregation sit in their coats.
DS1 wore an Emile et Rose outfit for his summer Christening but they don't seem to cater for colder weather.
I don't want him in a gown or a sailor outfit.
I'm open to putting him in a suit as long as it doesn't look too old on him.
He could easily need 18-24 month top half and 12-18 trousers by November so I would prefer somewhere which sells separates. Plus would ideally like a cardigan / jumper / jacket to wear with the outfit so he isn't cold.
Please can anyone help with my dilemma?

Repeats to self never ever post without previewing. My post is littered with errors (hangs head in shame and crosses fingers for a response).

I got a lovely white romper and cardi from Sarah Louise, my baby was only 9 weeks when christened. He was nice and toasty in that :)

Jojo Maman Bebe do nice seperarates for baby boys.

Thanks I'll start looking for an outfit