"After 2nd level, the amount of experience you need to gain a level goes up by 250 points
(i.e., you need +1,000 to reach level 2, +1,250 to reach level 3, etc.). After four levels
of this, the 'additional' amount increases to 500. About the point you'd expect this to continue, it gets extremely erratic on levels 11 (+1,500) and 12 (+500), then reverts back to +1,000 at 13th and 14th. Levels 15-18 add +2,000 each, and 19-20 start at +4,000 -- this implies the amount added doubles every four levels.
Except that level 21 adds +8,000, and 22 adds +3,000. After that little bit of weirdness, it does +10,000 for four levels, and +25,000 for the last four."
Here's a rough chart of the results; the first column is from the RAW, the second removes the 'jagged' parts, and the third assumes a simple doubling every time. The numbers on the latter two look funny, but the progression itself is easier to predict.
RAW Smooth Doubler
1 0 0 0
2 1,000 1,000 1,000
3 2,250 2,250 2,250
4 3,750 3,750 3,750
5 5,500 5,500 5,500
6 7,500 7,500 7,500
7 10,000 10,000 10,000
8 13,000 13,000 13,000
9 16,500 16,500 16,500
10 20,500 20,500 20,500
11 26,000 25,500 25,500 <- 500 XP difference
12 32,000 31,500 31,500
13 39,000 38,500 38,500
14 47,000 46,500 46,500
15 57,000 56,500 56,500
16 69,000 68,500 68,500
17 83,000 82,500 82,500
18 99,000 98,500 98,500
19 119,000 118,500 118,500
20 143,000 142,500 142,500
21 175,000 170,500 170,500 <- 4,500 XP difference
22 210,000 202,500 202,500 <- 7,500 XP difference
23 255,000 244,500 242,500 <- 10,500/12,500
24 310,000 296,500 290,500 <- 13,500/19,500
25 375,000 358,500 346,500 <- 16,500/28,500
26 450,000 430,500 410,500 <- 19,500/39,500
27 550,000 527,500 490,500 <- 22,500/59,500
28 675,000 649,500 586,500 <- 25,500/88,500
29 825,000 796,500 698,500 <- 28,500/126,500
30 1,000,000 968,500 826,500 <- 31,500/176,500
There are no clean mathematical formulas to approximate this, according to the curve-fitting service ZunZun.com. While my prior equation did serve to ... output the rates of increase, a lookup table or your own XP chart should serve better.
Curiously, this does highlight some interesting questions about the relative time per level, suggesting that a custom XP chart may provide a more consistent levelling experience for your players.
Edit.
Here's a quick and dirty reverse lookup that worked in excel 2008.
=VLOOKUP(A1,{0,1;1000,2;2250,3;3750,4;5500,5;6500,6;10000,7;13000,8;16500,9;20500,10;26000,11;32000,12;39000,13;47000,14;57000,15;69000,16;83000,17;99000,18;119000,19;143000,20;175000,21;210000,22;225000,23;310000,24;375000,25;450000,26;550000,27;675000,28;825000,29;1000000,30},2)
Best Answer
Do note that the 'Homunculus' you seem to be referring to is the D&D 3.5E or 5E homunculus. That particular form of Homunculus does not exist in the 4E ruleset.
So, to start off looking at the Homunculus you're talking about...
RAW
Wizards are the only ones who can create homunculi because they are the only ones who use magic in the 'right way' to be able to perform the ritual. As mentioned by you, this does not necessarily explicitly prohibit them from using someone else's blood in the ritual.
As Intended
Almost certainly 'No.' If a Wizard could make Homunculi for others....well....they aren't all that expensive to make. Why wouldn't a Wizard make a homunculus for absolutely everyone in the party?
Solution
Ultimately, this is your table. The DM adjudicates the rules and has full leeway to interpret them, ignore them, or add to them however they feel is necessary for the betterment of the game. If you want to say that a Powerful Wizard discovered an advanced form of the ritual that allows him to bequeath the creation on someone else by using their blood, and that he did so to gift such a creature to your player's character, then you can. You're the DM.
Since you are using the 4E ruleset, might I point you to the Arcane Familiar feat? This is the 4E way of getting a familiar, and is allowed to any Arcane Class, of which a Warlock certainly is one. All you have to do is take one of the options for familiars, reskin it into a Homunculus, and voila. Then you just make up some fluff about a Wizard, and you have a nice balanced addition.