Leonie nailed it - Bee does have epilepsy. However, her case is not usual, so PLEASE do not compare your DS's first focal seizure to Bee's epileptic encephalopathy.
You said he "started having focal seizures yesterday". What do they look like? Did the neuro actually SAY they were focal seizures? I ask, because a seizure can appear focal, but the definition is that it arises from a single point in the brain, and without already doing the EEG to determine where that focus is, he was speaking out of his hat. Complex motor tics or stereotypies can (and do) often resemble seizures and it requires testing to determine which one you are dealing with. At other times, like with Bee, there can be both - complex motor tics and The neurologist said that he would be intensely tested (why professionals use language like that when parents are already upset and unsure will never make sense to me). Likely, he'll have blood testing, EEG and MRI to determine if there is a reason to look further into "other" causes for the seizure or if it appears idiopathic. In all honesty (and in our experience), "intense" testing rarely happens over 2 weeks - unless it is inpatient and highly urgent.
An EEG may not show active seizure activity, but there is something called "post-ictal" or "inter-ictal" activity that can show evidence that a seizure has occurred recently. With that said, it can also appear totally average, in which case, it does decrease the likelihood that it was actually a seizure, but does not rule it out at all, as the EEG is only a brief "video clip" of the activity on the surface of the brain. Seizure can also arise from deep brain storming, but that is almost always associated with congenital brain malformations or traumatic brain injury. It is important to note that if the EEG is unremarkable, it does increase the chance that there will be a resolution of these episodes, unlike if the EEG is wildly abnormal, even in the absence of seizure activity.
The MRI will be to see if there are any malformations in the brain, issues from prenatal development, delayed myelination, lesions, signs of trauma etc. If he fell and had a hard knock as a little guy, but seemingly recovered fully, slight scarring can cause a "short" in the brain down the road. However, it is quite possible that nothing significant will be found in the MRI, either. Depending on the facility where he is having the MRI done, a second test may be done at the same time call an MRS or magnetic resonance spectroscopy. This would be ordered if they had reason to be suspicious that the seizure activity was being caused by an underlying metabolic disorder. It uses spectroscopy to determine if there are excessively high or low levels of particular chemicals in the brain (lactate, pyruvate, glycine, homocystine etc).
The blood tests will be looking for irregularities in his electrolytes (potassium, sodium, chloride, calcium etc.), blood counts, checking for some metabolic disorders, thyroid function and a few other things. This is routine in the first seizure follow up - it may seem overwhelming, but it is mostly precautionary.
The results of this testing can be helpful in trying to determine appropriate treatment for your son. On the bright side, well over 1/2 of children with seizures will see them resolve before adulthood. Try not to get yourself too overwrought about all this - by your description, he had his first seizure on Friday. It is Sunday now -2 days. You will be OK, he will be OK, and we are all here for you until you both are...