Well... of course, as soon as I post this I find a list from the Arbor Day order (or at least, I think it was that year's order). No idea which is which (which of the ones that came in this bundle actually survived), but could the third photo be a young Sargeant Crabapple? That's on the list, as is Eastern Redbud, Washington Hawthorn, dogwood (I know it's not that), Goldenraintree and crapemyrtle (I remember wondering why on earth they were shipping crapemyrtles to New England!!) In theory (meaning assuming they sent what they said they were sending), it should be one of those -- and the crabapple seems like the only one it could be? I'd be floored if they were shipping free Bradford pears (which is what someone on Name that Plant thought it was as well) -- aren't they considered more or less invasive plants in a lot of areas these days? So I'm hoping for Sargeant crabapple since I wouldn't mind having one of those for the birds (means the little guy can stay right where he is now, in the bird/butterfly garden).

Click here this thread should answer the question.

do understand.. nothing will be drought resistant for the first few years.. depending on size planted .. also.. keep in mind.. something that creates GOOD SHADE ... might EVENTUALLY limit what you can grow under it .... but that would probably be many years down the road ... a pic of the area.. might get some better ideas .... ken https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

Generally speaking, without closeup picture of the leaves, buds, bark and flowers, we can still most likely presume this is some kind of plum or cherry trees. These belong to the Rosaceae family of which there are 4,828 species in 91 genera, including apples. So, if those trees are the source of the problem, you'll have quite a time protecting him/her from this type of allergen because these are ubiquitous. (There is a 'Name that Plant Forum' on these boards). Have you seen the pup eating or chewing on plant materiel or anything else that might upset his stomach? Many kinds of plants can have a mildly to modest toxicity if taken internally and if they eat grass, they can vomit from that. It might be best to keep the pup out of the yard during pollination time, but sorry, I am not a veterinarian and have no idea if pollens from these trees can cause any of the symptoms you describe.