The drops are cycloplegic drops which will relax the muscles in his eyes to allow a very accurate prescription by objective means. What objective essentially means is there is a light called a retinoscope which is a handheld instument with a spot or a line beam of light which the optometrist moves across the pupil and moves in different directions and amounts with short and long sightness and astigmatism. When optometrists do their final professional qualifying exams they have to be able to do this very accurately to within a small degree of the final prescription. We rely on this measure with for example small children, some patients with learning difficulties, some patients with no English and no interpretor and in some cases of dementia.
The objective testing is refined by subjective questions such as asking if certain lenses are better than others, 1 or 2 or the red/green test. Some patients, often engineers!, answers are extremely accurate and enable the prescription to be finely tuned and with other patients we sometimes discount some test results and refer back alot more to the objective result. This is what we are trained to do and alot of the tests we do check if your answer agrees with one you gave previously to see if it is reliable and hence why a computerised programme can't do the test.
The retinoscopy result was +0.25ds with the optician today, but what he wants to see is if this goes up with the cycloplegic drops and by how much. If say it went up to +3.25ds your ds is doing +3.00ds of muscle work contracting the lens to see clearly which could give him headaches from eyestrain. It is important to look for any large differences between the two eyes which could indicate a lazy eye. 0.25ds is the smallest lens and not significant as far as a precription.
Normally children are longsighted (smaller length of eye) and glasses are not given for this, unless it is of a significant degree.Obviously as they grow the eye grows in length and they are no longer longsighted and some (like me!) become (very) shortsighted.
Children can fail on the letters just because they decide not to read them or don't try if they feel they may 'fail'.We can check by the objective methods ( the drops being an insurance for accuracy) if this is due to needing glasses. Sometimes we put a plano (no prescription) lens in front and they can read the small letters. Sometimes we may check them on a shorter recall to see if they are now reading the smaller letters.
The optometrist should refer on to the hospital eye clinic if any doubt exists. I would ask your ds's vision in each eye to be written onto his prescription - what we call 20/20 vision is the American term for British 6/6. The top 6 is the testing distance of 6 metres. The bottom number is the size of letter seen and gets bigger as the letters get bigger e.g. 6/60 is normally the big letter at the top of the chart.
Ideally all children should be tested with a line of letters (crowded testing)in preference to one letter (single testing) as this is more accurate and makes it easier to pick up a lazy eye/squint but some children do struggle. What I would do in this case is do the cyclo drops test and then bring the child back for a post cyclo test to have another go at the letters by a crowded method. If still failing to read smaller letters a hospital eye clinic referral would then be done.
Obviously I can't give more help over the internet,but hopefully this explains the tests we do a bit better. See how you go at the cyclo test, ask for the prescription and vision level on the letter chart and what the next step is. If you are still worried perhaps see another optician for a second opinion ( would have to be private if you go elsewhere and I would be clear with the other optician why you want this and if there is senior optometrist available as you have already had an nhs test), but you could just ask for a recheck with another optometrist who works in the same practice (which should not be an issue and wouldn't be charged) or ask the original optician to send a report to your GP with the results of the test and discuss regarding the headaches and whether a hospital eye clinic referral is indicated.
Phew.. Bit long. Hope I haven't confused you further.