Mistaking Kindness for Weakness...and apparently for stupidity!?

I will make this short because I've seen questions like this on here before...I have a boyfriend of three years, on and off, because of an illness I have for the second time I've needed him to be home to help me with my son (he is 12 years old) and to just be there in case anything bad happened, like I needed to go the ER or something...like I said for the second time he has acted like I'm inconveniencing him and not only did not come home but ended up staying out all night...I would love to be able to tell him I would rather be alone than with someone who is so disrespectful and irresponsible, but I can't find it in my heart to do so. Also, I just bought a car from a dealer that he is familiar with and whom he has done business with in the past. The dealer asked him to co-sign the loan since the dealer knew him but didn't know me...this is one of those second chance dealerships that finance in house. I'm afraid he will try to take my car if we were to break up...Any suggestions? I know I deserve better, but I was once told you can't help who you love...but I feel a little like a doormat. For the record he is almost 39 years old!