E.B.
I don't suggest trying to displace grief by replacement. Yes he would probably love the dog, but the grief is then only suppressed and very likely to affect him later in life in some way.
It also teaches a person that if someone dies, the grief process is not necessary, just replace them.
There are solutions to help and support his grief process resolve quickly and effectively, rather than suffer and be stuck in it.
You can give him a 2-3 doses over 2 days of homeopathic Ignatia 30C to help him resolve the grief. It works at a deep level, with no side effects. There are other remedies and higher potencies that are sometimes more fitting for a person which you can get from a homeopath if needed. But usually Ignatia is all a person needs, especially children.
Then he is not stuck in grief, nor 'robbed' or diverted from it's very necessary process for humans. :)
Be well,
Liz