If you replaced your HDD in your iMac with a new SSD and you try to boot, nothing is going to happen.
I am assuming you are getting something that looks like this:
That means your iMac can't find anything to boot off of because your new SSD is completely blank. (You just took it out of the box and exchanged it with the old one, right?)
Command + R doesn't work because, again, the drive is blank. You need to have a recovery partition for it to boot to. You won't be able to do an Internet Recovery either because your iMac is not supported. Internet recovery support didn't happen until 2011 and only was retroactive to 2010 Macs.
How to Install OS X on a new SSD When no the Recovery Partition or Internet Recovery Isn't Available:
If you or your son has access to another Mac (it sounds like it), have him download OS X for you (El Capitan is supported)
Create a bootable USB flash (you will need an 8GB or larger)
Install OS X. It will take about 30 mins so make yourself a cup of coffee.
Once you have done the initial setup and logged into your accounts/icloud/etc....
Then use "Migration Assistant" to move all your data from your old drive/external backup to your new SSD.