I would definetly second lindzy on that. Just do infant activities and stimulation, change activities, etc. But what he does seem normal to me as well, also remember different children have different characters.
Autizm connected to vaccinations in a few studies, so this is something to keep in mind... If you have not vaccinated, there are hardly any records on autizm among non-vaccinated children, who were not given any medications from birth. If you did vaccinate or had some meds, it is good to check if there were any sudden changes in his behaviour after the vaccination or after the medication was given.
However if hsi character was always like that, I would not worry much at this point and will just do activities with him, show lots of love, interaction, and enjoy him! He probably just loves his mommy so much that he is happy to look at you and laugh!