Young grafts should be treated closer to vegetable starts than to trees. A nursery bed for that first year, at least, is required. Although the tree will require different environmental and nutritional needs than an annual, they do need the same sort of babysitting. Consider water and warmth as the “fertilizers” of first importance. This latter point is all to often left out of the equation. This also means keeping the humidity up, especially if the union is not healed so well. (Drying out the scion may not simply kill it, it may be just crummy growth). Also, make sure it is the growing conditions and not the grafting procedure that is causing slow growth. This especially means aftercare for proper heal. When they are growing, give them the attention you would give to your little tomatoes, including mulching and weeding.
There will be a disconnect in how the baby is treated in this first year or two, and how it will receive nutrition as an adult. Later in life in a healthy system, it will do well with gradual release through organic matter, in conjunction with good mycorrhizal fungal connections. Now, in a bed that has been prepared with forethought (fungal connections partially in place) and living soil, good growth is possible. Even in this situation, some supplementation is helpful. A foliar application is great, but a sustainably derived fertilizer in that bed is a good idea as well. They benefit from a bit of nitrogen which, being easily available in a mineralized form is an easy one, as is K. But, P and other micronutrients really should have been developed in that living soil. It is when the quick fix is needed that growers turn to industrial farming methods like triple superphosphate, miracle grow and 20 20 20 .
Having done this quite a bit commercially, I find that that first year never really is all that spectacular for growth, whether that is seedling plants or grafted fruit trees. I think they are pissed from being moved around and violated, and they need to be at peace again. That second year is always awesome, for most. But, if the soil is a mess and it is too late to fix it, foliar sprays, watering on time, and keeping things in the low 70’s will give the best growth rates.
If the little guys are stunted in that first year or two, it will take a few more to get things back on track, in my experience. That said, a small tree is not always a poor tree. (Charlie Brown knew this) As long as it is healthy, slower can also mean denser and more resilient wood. I would put our smaller gritty trees up against the overly succulent fertilizer pushed trees any day of the week.