I have worked for two different commercial labs that both offer DNA testing for CF. Both labs charge < $400 for the DNA panel. Your best bet is to ask your physician to check with his lab to find out what they charge (direct to patient bill) for the DNA testing. Your boyfriend could be a carrier of the CF gene.
Here is what happened to the twins. His father has the gene. The mother of the twins also had the gene. That gave the twins a 1 in 4 chance of getting CF. Statistically, the following scenarios can happen:
Both mom and dad pass on the gene - baby gets CF.
Only mom passes on the gene - baby is a carrier, but does not get the disease.
Only dad passes on the gene - baby is a carrier, but does not get the disease.
Neither mom nor dad pass on the gene - baby does not have CF and is not a carrier.
With these scenarios, since you don't know if your boyfriend's mother has the gene, dad is the only way your boyfriend could be a carrier. His odds are 50/50 - either he is or he isn't. Only a DNA test can determine if he has the gene.
There is no other way to find out unless you don't want to worry about it. If you don't have the gene, there is no way a baby conceived with your boyfriend could have CF, even if he has the gene. It takes two copies of the mutation to get the disease. Again, if your boyfriend has the gene, but you are sure you don't, and you end up having children, the worst thing that could happen is he would pass on the mutation to your children. They would not have cystic fibrosis, but could be carriers of the disease. Hope this wasn't too confusing.