why should he receive special consideration? I don't agree with that. I would NOT ask for anything special from the teacher at all.
Special consideration provides equity for a person coming from unique circumstances, and this outreach avoids bias against them. Some may see what was suggested as a type of scaffolding, a temporary structure while one shows great gains in catching up with their new cohort. It is a form of support which in this case costs nothing to the school budget.
I... would not give him a certificate at home. That would feel like I was sending the message that my child could not handle not getting one.
On the other hand, it may send a message that there are many successes and many paths to those successes... he has taken an alternative path (which was not his first choice, but was the available option) and is successfully catching up... showing notable open-mindedness, bravery, and academic efforts. Moving up a grade is not a lark. If a family wants to reinforce his new placement as a positive challenge, not an overwhelming one, celebrating and providing a certificate may create a bond, happy memories, something for the scrapbook, something to look back on.
By analogy, when a family celebrates a child's summer birthday at home, does the child get the message that this is being done because the child couldn't handle not being celebrated in school (as may be done for those children whose birthdays fall during the school year)? That probably never crosses the child's mind. The outpouring of love and encouragement from one's family may take many different forms but is usually seen as positive and rewarding, rarely seen as compensating for a deficit. A kid who receives support will likely be able to lend support to others.