Uuummm... a RESOUNDING YES! I have never heard of a man who does not produce pre-ejaculate or ejaculate during sex. Even if he really is a 'fluke', he can still pass disease.
Yes, sweetheart, you could get pregnant.
Because:
A. Nobody is that in control at all times. He might be unusually horny one night and -- oops! Gee! That never happened to me before! Whaddaya know.
B. Pre-ejaculatory fluid is at body temperature. When a body-temperature liquid slips out, you can't feel it. (Ever had a tampon leak and have no clue?) He is not Mr. Total Control Of Involuntary Bodily Functions, he is just a guy who says he does not have pre-ejaculatory fluid, ever. Every guy has some pre-ejaculatory fluid. (Unless he does not have a Cowper's Gland, but I don't hear you saying that.)
C. You don't know what he has been doing before having sex. If he had had an unusually randy moment, perhaps he had been (let's say) having an orgasm in an alternative way than having it with you. Sperm can hang around a while in the urethra.
- - - -
Yes, you can get an STD. They do not come from orgasms. They come from the skin on the genitals and sometimes the mouth.
Sweetheart, he is not looking out for you. He is looking out for his own pleasure at the expense of the risk to you. Please at the very least, tell him no more wading without his Wellies, buy some condoms and say no unless one is on his eager little man. I may be making jokes but I am dead serious that you are being unwise if you do not.
In the meantime, get to the doctor and get on the Pill, or other good long-term method of protection like an IUD. While there, get tested for STDs. Wear a condom even while on the Pill if there is any chance at all he could give you an STD. (I.e., that would be, 100% of the time that you are single.)