Hi Suresh,
So exciting that you are walking with your nephew! The time you are going is likely to be wet and still pretty cold in parts - but green and beautiful! Last year there was a good amount of rain in mid-to late April in the area, with temperatures from freezing to 20C.
First, what clothes to bring. It is essentially the same year round, with just some modifications based on how cold or hot it might be.
Here is a simple system which includes three layers of clothing:
1) a base layer (shirt and pants) which is your wicking layer (wicks away any sweat); for April to May you may want to go with Merino wool shirts which will keep you warm or cool as needed and will dry quickly on laundry day; you will need two sets of clothing, one to hike in and one to wear in the evenings.
2) a layer for warmth, which is your fleece, puff jacket, or soft shell jacket; April to May can still be cold, so you don't want to skimp on warmth!
3) a layer for waterproof; you can choose a poncho or a rain jacket/rain pants combo; if you go with the jacket and pants, you will need a rain cover for your backpack
Second, for that time of year I would seriously consider waterproof boots. Wet feet can easily lead to blisters, which are no fun at all!
Third, for sleeping, if you are staying in the albergues (hostels), you will need a sleeping bag or sleep sack. Most of the albergues have blankets, so you could get by with the sleep sack. But if you 'sleep cold', you may want to bring a sleeping bag.
For the rest of your gear, here is my take on
What to Bring.
Buen Camino!
~ Nancy