I am sorry you are having struggles with this issue. As I read about this the red flags were popping up, because being really attached to one toy, or type of toy is a symptom of an autism-related disorder called Asperger's Syndrome. My daughter has it, so this is why it jumped out at me. Kids with this syndrome are extremely smart. Their IQ's are above average, sky high. They are usually more attached to their favorite toy than other kids are to theirs. It gets to an unhealthy level where it inteferes with other things in life, which may be why the daycare felt the need to take it away. My daughter's obsession happens to be horses. She will spend a long time just lining them up in a row. Kids with A.S. tend to prefer to play alone, or with their favorite toy, or along side other children, rather than interacting with them back and forth in conversation. They prefer to be with adults, older children, or younger children rather than with kids their own age.
They usually have sensory integration issues too, like being bothered by clothes, shoes, etc. Not liking to be touched, hate getting teeth brushed, hair brushed, etc. Not wanting to get dirty or wet, etc. Walking on their tip toes, covering their ears and/or crying over noises like the hair dryer, toilet flushing, vacuum, etc. OR they can't get enough sensory stimulation - still put things in their mouth after age 3, licks, tastes, smells things that they shouldn't. Craves water, sand, and playdough play. Doesn't seem to notice or care if it is cold outside and they don't have a coat on, touches things a lot that kids don't normally touch, (for example, my daughter HAS TO touch any railing, border, or edge around something everywhere we go) has no concept of others' personal space, etc.
Kids like this also seem to be very oppositional, and defiant, however they are not being "bratty" or acting "spoiled" they just literally can't cope. The world is so overwhelming and confusing to them that they have frequent "meltdowns" because they can't handle life and don't know how to explain what is wrong. They can be agressive, or just tend to ignore people all together in social situations. Sometimes they have poor eye contact, sometimes they don't.
They often have obsessive-compulsive or repetitive behaviors. (My daughter used to have to knock on every door 3 times before it could be opened, and had to be the one to turn on every light switch.)
Often kids like this will pretend to be an animal, (more than normal kids would) especially in social situations where they might be uncomfortable.
They don't really have a concept of time like knowing what yesterday means, or morning versus evening, and usually don't have a well developed sense of danger.
Thay also have a tendecy to wander off or run away.
That being said, not every kid with an autism related disorder like Asperger's Syndrome has all of these symptoms. These are just some red flags. The fact that your son has an obsession with tools to the point that the school is taking the screwdriver away because he played with it too much, and the fact that they want him tested for being "oppositional" and "defiant", those things combined are a cause for concern. I would seriously look into getting a doctor's opinion, and If your son has ANY of the other symtoms I have described, PLEASE get him evaluated for an autism related condition. The sooner you can get him help (occupational therapy, behavioral therapy, and if necessary, speech therapy and physical therapy ) the better.
I am not telling you this to worry you, but knowing what I know now with my daughter, I wish someone had pointed out the red flags to me sooner, because I could have gotten her help sooner.
Feel free to contact me if you have questions, and you may want to google asperger's syndrome, or PDD for more information.
P.S. I wanted to add that I have two other normal children ages 8, and 6. The difference in the behavior with my daughter who is almost 4, is night and day compared to their typical oppositional behavior when they were the same age.(All 4 yr. olds are oppositional to SOME degree) My other kids had favorite toys too, but it is definitley more of an obsession with my daughter with Asperger's.
One of the things that lead me to get my child evaluated was to observe her around other kids the same age and that is when I really began to see the difference standing out, even though I had been concerned for a while that she was different than my other kids were at that age. So, I just thought that it might be a good idea if you can swing it with your schedule, to go and just quietly observe him with the other kids his age one day.