how much does a weighted blanket weigh?
If I make it the correct weight (he's just under 30lbs so I guess about 3lbs?) then is it OK if he sleeps all night with it? Or is it only intended to be used for short periods of time?
The ideal weight varies from one individiual to the next and on the size/purpose of the blanket. Most OTs say to use 10% of body weight + 1 lb as a starting point. From there you work up or down depending on how much the person tolerates or craves deep pressure and the specifics of the blanket. Larger blankets that wil be used for sleeping can have more weight than smaller blankets used to wrap up with.
My son is 7 years old, 49 inches and just over 50 lbs. I recently ordered him a weighted blanket from Dreamcatchers. After discussing his size and needs with them, I ordered an 8lb twin sized blanket (43" x 75"). If I had wanted a smaller blanket (42" x 52") I would have gone with a 6 lbs blanket. Lap blankets should usually be about 1.5 - 3 lbs.
The Dreamcatcher website has a ton of great information about the size, weight, and materials for weighted blankets. You might find it helpful:
Is there any way you can keep the blanket you made, and just modify the weight in it?
The one I made is about 8-9lbs and very small. We actually have been using it for short periods of time right before bed, and it does seem to help him settle, so I guess it's worth making a proper one. I still feel like 3lbs would be too light for him though. He is... well, freakishly strong lol... I think 3lbs might feel like nothing to him. I wish I could test it out before going through all that trouble again.
So if I made a big one that's about 3lbs, would I be able to let him sleep all night with it? Or is that a big no-no? We co-sleep so at least I'd be right there but I don't want to do anything potentially dangerous. He really needs help staying asleep and I have to cuddle him almost the entire night every night, which is wearing on me (of course I like cuddling him but not for the whole night, I'd like to sleep too!!)
How old is he? The rule of thumb is 10% but we went much heavier because that was our child's preference. I wouldn't have done it for a baby or preschooler but for an elementary school age kid with good mobility I felt fine about it. We took it off after he fell asleep. As he got older he used it for the whole night and it has been fine. I don't think you've lost anything with the 8 pound blanket. You could always have him use it during the day - it could be kept in a "calm down spot" to encourage him to take a break as needed too.