My son is 3 years old and he was diagnosed with SPD, I was told to get him a 3lb weighted blanket to calm him at night.
What a miracle! Www.nonods.com offers the best price weighted blankets and their so soft. My son sleeps through out the night now.

I wanted to ask is it ok to give him melatonin?
Any long term effects?

We've been supplementing with melatonin nightly for over a year now since my son was about 4 1/2 yrs old. This was the only way he would go to sleep on his own. Give too much and it may have the opposite effect. I'd start with small does < 1 mg and see if that helps. I believe my son may also have a B6 deficiency that can also cause sleeplessness so that may be another area to look into.

I personally think most people take way too much melatonin. It's useful, and can help a lot of people, and while I don't know about it in young children, I do know that in general people take too much, and it can mess with them in the long run to take too much.

So, if you go with melatonin, make sure to not use too much. I personally use liquid melatonin instead of a pill. Liquid works better than chewable works better than pill for me; and I find with the liquid than as an adult I can use 1/20th of what is recommended on the bottle (which is what people expect to use), and have it work for me. Start with a tiny amount, the smaller amount that works the better it helps with sleep and the better it helps in the long run. One single drop of liquid is where I'd recommend starting, not one dropper full like liquid things recommend. It really can be enough - I vary between 1 and 3 drops of mine.

Generally the only long term effects of melatonin that I've heard of are from supplementing with too high of dosages though. It's not a thing where more is better. It's a thing where people think more is better because so many companies make things that are 5 mg melatonin, when less than 1 mg melatonin is actually preferable for most people. (There are rare people who should get up to 10 mg melatonin per night, they are rare.)