hiya!! as a switch (usually little) myself with a boyfriend who is also little, i've learned a bit that may help
first, id say love on him as much as he needs and then some. the little things like "good morning my little prince" or "goodnight little one, mommy loves you so much" go a long way, especially because littles tend to be more sensitive and need more emotional reassurance
even if you don't want to be his mommy/caregiver then you still should try to support him in anyway you feel you're comfortable with like helping him pick clothes to wear or coloring with him or letting him use his little stuff around you even if you aren't involved in it
and if you did want to be his "big", i know what has helped both me and my boyfriend is when the "big" talks in third person occasionally
not all littles are the same but 99% of the littlest i've talked to agree, and i've slowly found out more about what's common and what's not (i was worried for the longest time that i was just weird, but there is no weird in cg/l situations because it's just diversity)
i.e. stuff like "mommy loves you", "you're mommy lttle boy", "mommy needs you to please put your jacket on right now "
also, if he struggles falling into his headspace around you, and wants to fall into it and you're okay, make sure you provide a safe and comforting atmosphere for him. lots of cuddles and stuffies and comfort. all littlest are different and it's just a matter of you getting to know your little's wants and needs. maybe have that discussion sometime soon and just understand what he wants from a big if you plan to be his big, and maybe even set rules/punishments because i know that's always helped me fall into little space for sure
i hope this helped, my boyfriend added a bit too