I would say be very kind and gentle with them. Be sure to have your values match theirs. Support them with their decisions in life, even it if doesn't seem logical. Encourage them and let them know how their good qualities rule over their bad qualities. Do fun things you enjoy together. Let them know how much they mean to you.
ISFPs from what I've notice do really well with e-mails and texting. They seem to have a WAY easier time revealing themselves that way as they tend to be very private. Given time, they will find the courage to reveal their most inner thoughts and emotions to you. When this happens, be warm and attentive.
Be warned, as said before, it may take them a while to make a decision to start something with you. I believe ISFPs take longer to start a relationship with. Well, in general, ISFPs take longer to make decisions, so be prepared. If you can't live with that, then I would suggest just being friends. ISFPs like to move at their own pace and don't like others to hurry them along.
Finally, like him for who he is NOW and don't try to change him.
But Luke- you've stopped me dead in my tracks. I actually had understood the hints of some kind of conflict brewing between the socionics vs. MBTI, but I am sorry to say I don't understand how these P's and J's get switched just yet. If you can tell me pretty pretty please then why if you take the turbo test, it seems as if the results are square aligned with Socionics??? Also the description for ISFp sounds just like the Myer's Brigg's ISFP to me!! Gawd, this is painful as I am trying to understand and ISFP as we speak- by every definition this individual is ISFP all the way. I just don't understand it seems like we get along great! Obviously I'm entp. So what is the deal!!!!! I want duality. I have met an ISFJ in my lifetime who I also felt I got along with great. So you tell me.... what is the real deal...
I'd also like to add that the description of the ISFJ seems suspecting in that is is EXTREMELY like the description of my ISFJ friend. Also the ISFP description is EXTREMELY similar to that of my ISFP friend. So why do people dispute again? I'll have to do some research. ;p