K.R.
it sounds like he is not getting night-frights, so that is great (my Mommy-Rx for night frights is to have the opposite-gender parent go in to comfort the child ... because usually the things frightening them are things they cannot instinctively see how to trust, but the opposite gender usually can)
basic things like making sure he wears warm enough jammies so that if his blankets get kicked off he won't wake up from cold ... (eliminate as many understandable wake-up cues as you can)
per the asthma idea, get him a chiropractic checkup and adjustment ... random pain (especially neck pain if his pillow is holding his head awkwardly?) can easily break up sleep for all of us ...