Did the preschool teacher mention the idea without any further information? I would ask this teacher and the school(s) (counsellor, special education resource advisor, or similar staff person) for more information and possibly a referral. By schools, I mean both the current school he is attending and, if you are considering a different school with a special gifted program school. Schools often prefer to conduct any assessments with their own staff psychologists or have a list of preferred consultants. Some schools will not accept outside assessments.
Other places to seek out referrals:
- the public school district administration, if there are public school gifted programs
- your family doctor
- local gifted and talented parent support groups (you can try an on-line search)
- if you know any parents of gifted students, they may have already gone through the process and are familiar with local psychologists who will test
Since you haven't asked, I won't start a long discussion about the purpose of IQ testing at the pre-school or early primary ages. If you search this forum for past threads about IQ testing, you'll find quite a few discussions about when and if it is appropriate at the pre-school age, if you are interested.
I'll just mention that pre-K seems very early to test unless the scores are needed to enter a special gifted program or you need support to get special accommodations in a regular class. IQ assessments at the pre-school age are not as reliable as testing when they are older - about age 8 or so. There are schools and programs that require certain IQ scores for entrance to their kindergarten or primary school, so it has become more common to test earlier but it's not necessarily a good idea. Anyway, you'll find more information in past conversations in this forum. If you have further specific questions, don't hesitate to post them.