I'm with MWM, this sounds a lot more than ADHD. The eye contact - he might make good eye contact with people hew knows well, but not so good with strangers.
The "not getting it" with Santa - it also fits, with the social immaturity. Doing well academically, except for comprehension issues- that sounds like a possible language issue, even if he's talking normally. It comes down to communication, especially the subtle "between the lines" communication.
We've handled things in a different way. Gifts from Santa were always unwrapped and placed either in or under the stocking. All other gifts could not be opened until after we got back from church on Christmas morning.
A kid nagging to open gifts - the gifts would be removed to get them out of temptation's way. Out of sight, out of mind. Not confiscated, just put elsewhere until it was time to open them.
As for appreciating gifts - we have more to our family ritual. Gifts tend to be opened one at a time (this year we did two at a time, there was a crowd) and before the gift is opened, the receiver has to give the giver a kiss and a hug and say, "thank you". Appreciation of the gift is ingrained into us all from early on; everyone else present would give a kid heaps if they were ungrateful or unpleasant about it. They would find no more gifts handed their way until they mended their manners. It could stop all proceedings until the problem got resolved or they left the room.
We also try to not spend too much money on individual gifts, although there is some variation according to who is giving the gift, what they can afford and how close they are to the person receiving the gift. For example, mother in law last year gave us $1000 to be used to pay for air fares to go to New Zealand. We were very grateful, that is exceptional. This year she gave us two bath towels. OK, they cost $50 each (she accidentally left the price tags on!) but they are lovely towels, we're very happy with them. I know she spent more than that on difficult child 1 & daughter in law, she bought them bathrobes which cost more than $50 each. It's just how it worked out this year.
Your difficult child's bio-father may have all those problems because he has a hereditary and untreated problem of his own. There can be many causes, including pure chance. I certainly agree with getting the evaluation. Also if you can, dig out the old test results. What is really useful is to look at the sub-scores, especially if there are big gaps between the highest and lowest scores. Which ones are high and which ones are low, can be vital.
A lot of what you describe sounds so very familiar. It certainly sounds sufficiently serious to deserve good answers.
Marg