DH could have genetic testing to see if he has inherited the Fragile X. If he hasn't inherited it, he can't pass it on to your kids.
You probably know this, but fragile X syndrome is caused by a 'dynamic' or expanding mutation. A segment of the gene expands between generations and if it gets too big, then the baby can have issues. It is only when the Mum has a semi-expanded gene and passes it on that the gene may expand further and the child may be affected. If Dad has a semi-expanded gene, the same size gene will be passed on to the child. So even if your DH has inherited the family fragile X gene, his kids will only inherit the same sized gene.

