It really depends a lot on the severity of his disability & this child's individual personality. I used to work in group homes with people with developmental disabilities. The first question is the level of mental retardation that accompanies the Down syndrome. People who are only mildly mentally retarded are very high-functioning, personable & largely independent. In that circumstance, you would interact with them in the same way you would any child client. Do a pre-session interview; check in with them during the session and alter techniques as needed. However, people who are severely mentally retarded have difficulty with all tasks, including ambulation & communication. They may have trouble getting on & off the massage table. They may still be wearing diapers. Sometimes they can't talk; they wouldn't be able to answer questions or communicate their needs during session, meaning you'd have to rely heavily on your intuition. If this is the case, I recommend the parent be in the room with you during the massage, so they can translate the client's needs for you. (Parents become adept at knowing what their children want, even if words aren't used.) There are also lots of ranges of abilities between these two extremes. People with Downs differ greatly, just like all people differ greatly. The challenge for you as a massage therapist is to customize your session based on their level of ability.
I recommend asking the parent to describe in detail this child's disabilities, so you can be as prepared as possible before the appointment. Does he talk? Is he easy to understand or does he have a speech impediment? Does he walk? How does he respond to meeting new people? What are his diagnoses in addition to Downs? (Some common co-diagnoses will be hearing or visual impairments). Have the parents ever massaged their child? If yes, what techniques does the boy seem to prefer? Are there any physical disabilities you should be aware of? (Mental & physical disabilities sometimes go hand-in-hand). Does he have any health problems (prior surgeries; other health conditions like asthma, etc.)
Once you have these answers, we might be able to give you more specific advice. No matter what this child's level of ability, massage can definitely be beneficial, and I think it's wonderful his father wants to offer his child professional massage.