My daughter is a beautiful, mature 14 year old who met a 15 1/2 year old boy at a sports function 2 months ago... since the get-go they both have professed a very strong connection for one another and the boy has told her he loves her profusely and even wants to marry her one day, mostly through text and phone convos but also in person to her face. He has never tried anything with her sexually and they have only kissed a few times... My daughter believes every thing he says and I actually talked to him and he seems very sweet and sincere, but I'm not sold even though he has pleaded with me to try and convince me that he is a good kid and loves her, and has no one else in his life he is interested in. I'm not sure if I should encourage the relationship because... it seems that his family situation is not stable and he will lose contact with her periodically for a variety of reasons, grounding, grades, loses his phone, etc... usually always as the weekend approaches, he will reappear with some reason for not contacting her for a period and the same pattern of behavior ensues. My daughter believes in him and says that she thinks he really loves her and is not just playing with her, because that would require too much effort on his part to keep it going and who has time for that... she believes he has issues at home sadly out of his control and may even be somewhat emotionally abused... he's moved 6 times in 5 years, mother's been divorced 3 times, had a live-in boyfriend that just left her, 5 kids from 2 marriages, possiby drinks, etc... but she does not believe it is his fault and truly feels a strong emotional connection, not just physical to him and they never argue and can talk for hours about everything under the sun, and does not want to abandon him over something he can't control, especially if he is vulnerable, however I want to advise her in the best way I can on how to go forward or not in this relationship, but I need some advice... Please help

There are a couple of things in your post that would worry me,firstly what would seem to be your daughters caring nature,she wants to help him,protect him,that's good,but a lot of pressure and worry for a 15 year old.

His family background is shakey,I would not blame him for that,but what would worry me is how he copes with stress and different situations, he only knows what he has learned at home,and perhaps he will or is leaning on her too much..

If it was me and my daughter I would moniter the situation very closely,and keep open and honest communication with your daughter.

Encourage her to spend time with her friends,have sleep overs etc...

I would be inclined not to encourage the relationship only because this young lad is carrying a lot of emotional problems,( from your saying about his background), and I would fear his influence over your daughter.