For meditation to work you have to build it up, begin when cam and do it regularly (at least once a day, before bedtime is good) because you can NEVR begin a series of meditation therapies when you most need it - you have to already have the skill and practice.
Aromatherapy - if he will let you touch him, massage is good, even if all you do is massage his hands or his feet (or shoulders). difficult child 3 got a bit sunburnt and I used the excuse to massage in some coconut oil scented with sandalwood. I also have some rose-scented coconut oil (the coconut smell has been removed) and this is a better scent for relaxation, although a bit 'girly'.
Lavender is brilliant for relaxation. It's good in a massage oil, or a few drops in bathwater is good for relaxation. An oil burner with lavender is also good, but make sure it doesn't aggravate him - this can happen with some people.
For boys, lavender is a good choice. Other good oils to include are rose (a small amount can be slipped in with lavender), sandalwood (go easy on it), patchouli. For a more stimulating mix, use a citrus -scented oil like lime, bergamot (also OK for boys) and neroli. The more floral oils need the woody notes for boys to not feel self-conscious. Also, don't mix more than two or three oils. Use a vegetable oil base, with only a few drops of essential oil to about 50 ml of vegetable oil (as a massage oil). For the bathwater - don't add more than about five drops. The same with the oil burner. You will soon work out what you and he can tolerate. And do try to get pure essential oils, avoid anything labelled "scented oil", "massage oil", "fragrance oil". The woody oils (patchouli, sandalwood, ti-tree or melaleuca, eucalyptus) are also antifungal and antiseptic. The last two are also good for chest and head colds, but go very easy on them, especially eucalyptus - too much can burn your nostrils and they're toxic if swallowed. Ti-tree oil is almost magic on skin inflammation and brilliant on burns). Rosemary oil is great for the hair - 1 part rosemary oil to 4 parts almond oil makes a good scalp rub.
Pure essential oils are very expensive, depending on how rare they are or how difficult to extract. Pure rose oil (attar) is unbelievably expensive. Lavender is quite inexpensive - about $20 for an ounce or so. Eucalyptus & ti-tree are even cheaper, because it's so easy to extract and down here we have forests full of it - you can see the air shimmer with the blue haze of oil. We've even been playing with extracting some of it ourselves at home, just for the exercise. Keep essential oils in a cool dark place. Browse the internet to get a feel for prices, so you don't get ripped off or sold inferior oils.
Basically, I would suggest you go very carefully and slowly, he may not cope with strong smells. A tiny amount externally is safe, so start very small and see how he goes.
Marg